In D&D 4e, an encounter's XP budget suggests one standard monster of the player's level for each party member.
Does it work well to spend the XP budget on smaller number of high level opponents, or a larger number of low-level opponents? What problems arise?
Examples for five level 5 PCs (~1,000 XP):
- 10x Lvl 1 creature (10 x 100 XP = 1,000 XP)
- 40x Lvl 1 minion (40 x 25XP = 1,000 XP)
- 11x Lvl 8 minion creature (11 x 88 XP = 968 XP)
- 3x Lvl 8 creature (3 x 350 XP = 1,050 XP)
- 2x Lvl 10 creature (2 x 500 XP = 1,000 XP)
- 1x Lvl 14 creature (1,000 XP)
It does not work well to spend the XP budget on a lot of much lower level opponents or a small number of much higher level opponents.
The ability to hit the creatures also matters and also greatly depends upon the level of the characters. You should attempt to keep the levels withing 3 levels of the party, otherwise you will quickly make things too hard or too easy.
I've honestly found 3 to be the magic number in my games, otherwise the players get easily frustrated that they are constantly missing the high level monsters. It is better to use an elite or solo monster if you want to make it a challenge.
I suggest looking at the template designs on page 58 and 59 of the DMG.