RMXP: Maps - up to 999 x 999 tiles.
Tileset - 8 x (very large number) tiles (each 32 x 32 pixels).
Up to 999 different tilesets.
I don't think there's a limit to number of resources - Essentials has at least 3000 sprites and still work.
In-game switches and variables - up to 5000, although this can be easily increased with a little scripting knowledge.
However, if you use the default systems, there is a limit of 999 on most things (enemies, weapons, skills etc). I'm sure you won't be using them for a Pokémon game, though.
And RMXP has one major thing going for it - the existence of a virtually complete Pokémon starter kit.