+2 EL for each doubling of number.
CR 8 monsters.
1 = EL 8
2 = EL 10
4 = EL 12
8 = EL 14
16 = EL 16 (not actually challenging to PCs)
etc.
Note that you do reach a point where they become ineffectual. They can't hit ACs, or PCs will make their saves 95% of the time, or the monsters will fall to a single blow or area of effect spell.
According to the DMG, hand out XP for each individual moster as appropriate to its CR. So level 16 heroes facing 16 CR 8 creatures get 0 xp, since anything 8 CRs below your level is not worth any xp.