(This article arose from a discussion I had with my email buddy Rob Kuty.)
The CR and EL system in the DMG is based on a party of four PCs. For groups with more than that number, DMs may have a hard time balancing encounters. Too often they end up too weak (and a cakewalk for the PCs) or too powerful (and often kill one of the PCs), which makes the game less fun. So here's some advice on how to create encounters for larger groups of PCs.
This is not a mathematical analysist of the CR/EL system and I don't want it to be. And in any case, it doesn't have to be--the point of the CR/EL system is that you can design encounters that are easier or harder challenges and reward the players appropriately (the math is on the paying (XP) end, not the working (CR/EL) end).
So, when designing encounters for larger parties, there are two premises to keep in mind:
Premise: A group of 5+ characters of level X is more powerful that
4 characters of level X.
This one's obvious. A group of 5, 6, 7, or more PCs is going to have more hit points, spells, and skills at their disposal than a group of 4 PCs.
For example, there's what I call the "hit point heat sink." A larger group of PCs has more hit points than a smaller group and have extra characters to compensate for fallen characters (a group of 4 that drops to 3 is suddenly operating at 75% capacity, while a group of 8 dropping to 7 is still at 87.5% capacity). At low levels, a party of 8 including 1 cleric stays up a lot longer than their level would indicate because the cleric can cast cure spells every round, bringing people up to full or nearly full hp. Plus, enemies can only target so many foes, so damage ends up spread out over many characters, so each character takes less, so characters are less likely to fall.
Premise: A larger group of PCs has more tactical options than a smaller
While a group of 4 PCs might have a hard time setting up flanking to allow the rogue to sneak attack, a group of 6 characters has many more opportunities for that simply because they're threatening more squares (giving the rogue more places to stand where she can flank). Likewise, a group of 6 PCs that includes 1 wizard are going to have an easier time surrounding and defending that wizard compared to a group of 4 PCs that includes 1 wizard.
Conclusion: Larger parties are better able to handle encounters of
a particular EL than smaller parties.
While this would seem self-evident, I'm stating it because there is one false corollary that can be drawn from this conclusion, partly based on the rules for determining EL:
False Corollary #1: Doubling the monsters increases
the EL by +2, so 4 if PCs can handle an EL X encounter, 8 PCs can handle
an EL X+2.
The CRs of creatures are built on the idea that a party of 4 PCs is going to be fighting them, and the CR of the monster determines if it's an appropriate challenge for PCs of a certain level. For example, a barghest is CR 4 ... an adequate and appropriate challenge for 4 4th-level PCs. 4th-level PCs ought to have about 5,400 gp worth of gear, which means that all of the fighter-types ought to have a +1 weapon, which would allow them to get through the barghest's DR 15/+1 (secondary fighters like the rogue and cleric may not have +1 weapons and may have a hard time facing it in melee).
Now if you use the false corollary above, you might think that if 4 PCs of level 2 can handle an EL 2 encounter, 8 PCs of level 2 should be able to handle an EL 4 (2+2) encounter. The problem with that idea is that 2nd-level PCs are only supposed to have about 900 gp worth of gear ... not enough for a +1 melee weapon. That means that those PCs are going to have a really hard time bypassing the barghest's DR 15/+1. Also, the barghest's attacks are +9 melee and +4 melee, which means it's probably going to hit at least once per round and deal at least 6.5 points of damage per round (average bite damage) per round, which means that in 3 average rounds there's a good chance it's taken down one 2nd-level fighter (who probably has about 18-20 hit points). Since the PCs have a really hard time getting through its DR, that means the fight will last at least three rounds and therefore at least one character is going to drop, and then it's just going to repeat every three rounds until all of the primary fighters are down and it can start going to town on the rogues, clerics, and wizards in the group.
The scenario is even worse when you have a creature with an area attack, like a young white dragon (CR 3) which does 3d6 to everyone in the cone (it can take out some 2nd-level characters even if they make their saves).
The biggest problem of using one-creature CR-appropriate encounters for a large group is that those monsters tend to be big bruisers capable of instantly killing a low-level character with one hit. A fight where the PCs are hoping to kill the monster fast because it's killing a PC every round isn't a fun fight.
So don't use the "double the numbers = +2 ECL" rule for creating encounters for large parties.
[While I'm briefly on the subject of dragons,
I should rant about this thing that not a lot of people know: Dragons are
intentionally undervalued for their CR. Why? Because dragon CRs are set
assuming that the PCs known the dragon exists and are planning to fight
it. "PCs shouldn'st stumble into a dragon's lair" is the argument. While
that may work for the big dragons, unfortunately it doesn't apply to the
smaller ones, whose CRs are still way too low.
For example, a young white dragon is CR 3, but it has 9 hit dice, AC 18, +11 base attack, and a 3d6 breath weapon. By comparison, two ogres are EL 4, have 8 hit dice between them, AC 16, +8 base attack each, and no breath weapon.
They're the only monster in the book that has its CR set up like that, and (even worse) nowhere does the book tell you this! See the problem? So when you use dragons, treat them as if their CR were at least 1 higher.
And here's another important false corollary to know:
False Corollary #2: One big opponent is better
than several small ones.
This may be true for smaller parties, but much less so for larger ones. While a single wiz4 might be an appropriate challenge for 4 4th-level PCs (who still might have an easy time dealing with him), a single wiz4 is a cakewalk for 6 or 8 PCs ... they can easily surround him and force him to take multiple AOOs every time he casts or suffer multiple readied actions every time he casts defensively. And then when you consider the PC rogue is basically guaranteed flanking, and the PC wizard can sit back and actually counterspell of the off chance the enemy manages to successfully cast a spell, and realize that the wizard gets to act once each round while the PCs get to act 6 times per round, you see why single opponents are poor choices for large parties.
So having one enemy instead of several is a bad idea for large parties. Those odds, no matter what the CR (as long as the CR is within about 5 of the ideal CR, we're not talking 7 1st-level PCs vs. a wiz20), are going to turn bad against the solitary creature.
What you really ought to be doing at low levels is using more weaker creatures instead of one large one. Yes, it's a pain from the point of running the fight (you have to control more monsters) but it works out better in the end. So use larger groups, and solitary monsters should have buddies, especially with a large group of PCs as enemies.
Another way to look at it is to skirt the edge of false corollary #1. If you have 8 PCs, that's almost like two groups of 4 PCs. If they're 1st level, make two EL 1 encounters and combine them. If two CR 1/2 orcs is a EL 1 encounter, four orcs is about an appropriate encounter for 8 PCs. If you need a EL 2 encounter for your 8 PCs, make two EL 2 encounters for four characters and combine them (3 orcs is EL 2, so 6 orcs is about right for eight characters, and that way almost everyone has an individual opponent, but there's enough PCs left over to allow the rogue to gang up on one and sneak attack, etc.). Overall the 6 orcs may be EL 2, but it's really like two separate EL 1 fights because the party is large enough (and the PCs are more likely to survie +3 melee orcs that do 1d12+3 damage than they would a +8 melee ogre that does 2d6+7 damage, and is the same EL).
Some last points to remember:
It's OK to have weak encounters.
Not every encounter has to be a tough encounter. See DMG table 4-2 on page 102, it gives guidelines for how many encounters should be easier or more difficult than normal. If you have it skewed in the weaker direction just to make sure you're not killing characters unfairly, that's OK.
Basically, a larger party is going to be able to more handle EL-appropriate (for a party of 4 characters) encounters before having to rest. If a group of 8 1st-level PCs can take 8 CR 1 encounters before deciding to take a break, that's OK. One, remember that an EL-appropriate encounter is supposed to use resources (spells, hp, ammunition) but not really be a serious life-threatening risk, so don't feel bad if they breeze through some encounters. Second, remember that the PCs will be dividing XP eight ways instead of 4 ways, so they'll progress more slowly than the equivalent 4-person party going through the same encounters. So they can handle more and rest less, but their progress will be slightly slower than a standard party and so their levels will always be a little behind, but their numbers will make up for it
Use a mix of opponents to increase EL.
When you want to elevate an encounter from "challenging" (EL-appropriate) to "very difficult" (or in any case from a weaker category to a stronger one), try increasing the power of one of the monsters in the group. For the orc encounter example, it's often as simple as replacing one of the orcs (which are normally warriors, and CR 1/2) with an orc with a 1st-level PC class (which would be CR 1). Using table 4-1 to determine the EL with this change, a CR 1 + CR 1/2 encounter is EL 2. So 2 orc war1 is EL 1, but an orc "grunt" war1 and his "boss" ftr1 is CR 2. For eight PCs, make that two bosses and two grunts, or even use a clr1 instead of one of the ftr1 bosses.
Once you've found an encounter size that works for you, it's best to handle tougher encounters by doing the above: pump up one guy a bit rather than adding a bunch more guys. In other words, if the ideal monster group is about the same number of monsters as your PCs, more monsters isn't necessarily better (it's more work for you, you're more likely to forget someone's actions, etc.). Just take one of the monsters and make it tougher. Doing so also prevents the PCs from being able to judge how tough the encounter is (before the fight starts) just by counting foes: 12 orcs are obviously more trouble than 6 orcs, but 6 orcs (3 of whom are fighters) look much the same as 6 orcs (all of whom are warriors), and so the PCs might wade in expecting an easy fight and have to revise their estimation after round 1. While the DM's job isn't to trick the players, it is a good idea to keep them on their toes and keep them from getting too cocky.