I have found that once you get around 7th to 9th level, you cannot really push the characters very hard with a single encounter without a nearly unacceptable risk of a TPK.
It also depends on what your throwing at them and how quickly the players can kill them, as well as a number of other intangibles.
Here are some telltale signs that your monsters wont really cause your players any serious difficulty absent using unholy numbers of them.
- If the monsters only hit on 18 or higher, and only inflict damage on successful attack rolls BEFORE AC buffs
- If the players can drop them in one hit
- If your players can render them entirely ineffective
If any of the above are true, you could probably run 9 or more consecutive combats without making much of a dent. There are complicating factors to consider as well. Cannon fodder that can only eat 1 attack before dying may still be viable if they can hit often. But if they are melee based an you have players with Cleave or Reach + Combat reflexes, it wont matter.
Other than that, a good guideline may be 1 combat encounter per 3 or 4 character levels. This assumes that the encounter meets the following criteria.
- The opponents can inflict enough damage to be a threat to your front line combat guys
- The opponents can guarantee their own survival for at least 5 rounds of full on combat
- The opponents can function reasonably close to their optimal potential for most of the time they are alive.
For inflicting damage, it can be breathweapon / save for half based, or it can just be a good enough attack bonus to hit on about a 15 or greater when you have 4 or more opponents. For survival, I dont mean for every opponent to still be standing after 5 rounds, but the primary threat should still be alive (if it is a single BBEG supported by fodder) or you should still have a few monsters kicking around if its a small squad of semi durable monsters.
The last point is somewhat important. If your spell caster can routinely neutralize most of your badguys with 1 or two spells, you need to rethink how you stage combat. I recently had a fight with a dragon turn out to go much easier than expected simply because it failed a save vs Slow. Suddenly, I could no longer use flyby attack, or full attack. Being limited to a single attack made the dragon essentially a non threat. I tried to retreat, but ended up dying due to having to eat a few AoO followed by some hard hitting ranged spells.
I am new to running games for characters around 8th level and higher, and still figuring out what kinds of encounters work for my group. I have come to a few conclusions though for that level range.
1) No Solo monsters without support. Unless I want to go for something overwhelmingly beyond the players means, it just never works. The players can put out too much damage on a single target.
2) No 1 Hit Fodder. The fighters take them down too easily with just iterative attacks. On top of that, the players have a whole lot of area of effect spells. Its one thing if they only have 1 or two fireballs to play with, since players tend to hold back on their top guns. But if they can toss out 5 or 6 of them, they will cut loose.
3) Mix it up. While you can throw together an encounter with nothing but 2 or 3 brutes that have good attack bonus and good damage output, you run the risk of being neutralized too easily. You should have at least 2 ways to cause harm to the players. Half melee and Half archery will do. Half direct damage and half 'debuff' / battlefield control is also good.
4) Include reasonable workarounds for player defences. Try to include at least one counter to a trick your players like to use. If they go for invisibility, than have a reasonable, and plausible counter measure handy, such as a monster with Scent, or a Dispel Magic. Do not include a counter for every trick, and do not always counter the same trick. But have at least one or two counter tactics available. Failure to do this will result in your players winning every fight using the same tactic.
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