The spell Banishment says it affects:
As many as 2 Hit Dice of creatures per caster level can be banished.
I'm fairly new to Pathfinder and still haven't quite figured out what a spell means by "# HD of creatures".
How do you figure out what a creature's number of HD are?
Color Spray and Sleep have similar restrictions to their effects.
Best Answer
Monsters have various numbers of Hit Dice. This is information that in general the GM knows and it's listed in the Bestiary (and the SRD).
In general, class levels are 1 HD each, so your level 8 buddy has 8 HD. Things like orcs and zombies just have one or two HD; devils and demons and stuff you are likely to banish will usually have 8 or more.
In the monster's listing, it's the number of dice listed under their hit points. For example, the Demon, Vrock has listed in its stat block "hp 112 (9d10+63)". That's 9 Hit Dice (of size d10). Of course, once you overcome SR and their saves, that is. So if you're level 9, you can banish 2 Vrocks.
In general with Banishment the way the math comes out is that you should be able to target 1 boss, a couple major threats, or a bunch of nuisance threats.