If I understand your questions correct.
There are no "rules" regarding how many events sheets you should use. The reason why it can be a good idea is solely for organisation.
Imagine you have a game with 2000 events all made in one event sheet and you are trying to find a specific event. It can take quite a long time to find. Especially if you are not 100% sure what you called it.
So a good practices is to organize your events into event sheets, so you know where to find them. Since the amount of event sheet doesn't impact the performance of your game you can add as many as you like. An example of how it can be good to organize your event sheets could be something like this (This of course depend on the type of game you are making).
Map generation (All events that you use to generate your map.)
Map generation functions (This hold all functions that might be used. The benefit of adding functions to there own sheet is that they will never run per tick. So its both easier to find them, and you don't get them mixed up with normal events and triggers.)
Player control (Hold player movement, attack, death, experience, level up etc.)
Enemy control (The movement of Enemies, enemy attacking, death etc.)
Enemy AI (If you need some more advance AI for enemies it might be a good idea to separate that from the enemy control, so it doesn't get to big.)
And as with the map generation example it might be a good idea to add a Function version only for functions for player and enemy as well. But in general whenever you think that you can group functionality together into an event sheet, its probably a good idea.