The limit is set by you and your time, not by pokémon essentials and it really depends on how you set the game to be.
If you balance it the same way as Original Pokémon games, you'd end up having your players have level 100's by three regions.
From there you can decide whatever to just stop there or continue by increasing the limit or rebalance it in a way that the levelling will be slower.
The maps are limited to 999 if I remember correctly but I'm pretty sure that, should you reach this amount, there are workarounds to break this limit (In rpg maker XP, there was, not sure about essentials).
