Hmm...
On a test map, have a set number of enemies and check the lag there. If it's not laggy, add a few more enemies. If it's too laggy, remove a few.
Keep doing this until you find that you can't add anymore enemies that the lag is undesirable.
There's your magic number of enemy events you can have at one time.
Compromise with RPG Maker XP on that. Or your computer, even~