Pathfinder - Encounter Design
This article assumes that you have already read the section on Encounter Design in the Pathfinder Roleplaying Game core rulebook, or on the Pathfinder SRD. This is intended as an expansion upon and supplement to the encounter design rules published in the rulebook.
A Note About CRs below 1
When counting CR down from 1, the CR progresses by fractions: 1 > 1/2 > 1/3 > 1/4 > 1/8. If a threat's CR ever falls by "-1", and would fall below 1, move it one step along this track.
Example: NPC characters' CR is their level minus 2. A level 1 Warrior is CR 1/3, while a level 2 warrior is CR 1/2.
Scaling Encounters for More Than 4 Players
If you're anything like me, you have almost never fortunate enough to run a game exactly 4 players. My ongoing games typically include 5 or 6 players. While I certainly enjoy having my friends around the table, larger parties provide some mathematical challenges when designing encounters.
Why More Players Requires Adjustment
Adding even one player above 4 can be hugely destabilizing. One extra player can be an extra person to flank with, an extra save-or-suck spell every round, an extra chunk of damage, or a whole mess of buffs (I'm looking at you, 5th-man bard). Any good party is greater than the sum of its parts, and adding extra players does not change that.
Even from level 1, extra players can do a lot to upset the balance of encounters. As of this writing, I am running Way of the Wicked for a party of 6. Without going into any real spoilers, the party begins unarmed and unarmored in prison. Several of the early encounters pit the party against two guards (human warrior 2), which makes for a CR 1 encounter. For 4 characters, this would be a scary fight without normal gear. With 6 players, this encounter is a joke, even without weapons or armor.
To handle scaling, the rulebook suggests adding 1 to the party's "average player level". I find that this rule does not typically solve the issue of additional players. As an example: The "Advanced Creature" template adds to the stats of a creature without cosmetically changing it, and increases its CR by 1, which should compensate for 6 or more players according to the rulebook. Any solo monster (a dragon, a beholder, etc.) would still be hard pressed to defeat 6 well organized players. This is due in great part to the huge difference in the number of actions which the players take compared to the number of actions which the monster could take. A reasonably well built party of 6 with decent initiative rolle can easily kill a solo monster of their CR (or sometimes several CRs higher) before it can take a turn.
Determining Experience Budget
The official "Experience Point Awards" table lists an estimation of experience to be awarded to each player depending on how many players are in the party. This is a rough guideline to save you the trouble of dividing, and I generally choose to ignore it. It also assumes that you do not add additional threats to the encounter to compensate for additional characters.
Instead, use the following table to determine how much experience to allocate per player per encounter. Note that the #/level column specifies the number of encounters needed for the party to reach their next level at the specified game speed.
Spending the Experience Budget
There are two ways to spend the additional experience budget added by expanding the party: Buff the existing threats, or add new ones.
Buff the Threats
If you get to the table and find yourself with extra players, there is no shame in throwing the Advanced Creature template on one or all of the enemies in an encounter. The players likely won't even notice, but the encounter should hopefully still be challneging.
Note that this approach can have very poor results for solo monster encounters. Because the party has so many more turns than the monster, it will have trouble defending itself, regardless of improved stats.
Add new Threats
Multiple threats in an encounter is a very complex balance between too many diluted threats and so few threats that the players can completely overwhelm them with sheer numbers. For normal threats, I find that two monsters of 2 CR less than the party's level is generally an ideal balance. The monsters are still strong enough to give the party a moderate challenge, and won't fall apart every time a player sneezes. If you have 5 players, you may be able to get away with applying the Advanced Template to one of the enemies. If you have 6 players instead of 4, you can just add a third monster.
In the case of solo monster encounters, you can scale the encounter by adding minions (a bunch of low level monsters), Minions (note the capital M), or environmental hazards (traps).
Using one or more lower-level monsters as minions is often problematic as it leaves you with a fairly small experience budget. This then leads to groups of low level enemies who can't hurt the players. This means that the encounter is essentially unchanged by these minions presence. Instead, take a look at my rules for Minions.
Traps can serve very well to scale an encounter. Many traps have relatively high attack rolls and damage for their CR because they only take effect once. For example, the CR 1 Arrow Trap has an impressive +15 attack bonus, which is high enough to have a good chance of hurting players well into mid level. The CR 1 Pit Trap has a similarly high DC 20 reflex save to avoid. While 2d6 damage might throw an encounter, falling 20 feet can take a character out of combat for a round or two, which goes a long way to handle the issue of too many players.