Say someone is making a horror game or an adventure type game with loads of different maps that you have to explore, some massive, some small, some medium. Something like “Paranormica” does.
Say I make about 4 small maps at the start, then continue adding more features and maps, say about 30 small maps at the end, is that too many?
Anyway, here’s a poll if you don’t want to answer fully, just looking for advice!
0 voters
If the game is round-based, 15 maps is an ideal number. That number gives you time and comfort to add detail to pre-existing maps. It also depends how small the maps actually are.
I would say 15-18 is an ideal number. But if you want to continue the game i would say make a new chapter like piggy did. Hope this helps! 
-Slime
Your thread’s title and poll ask two fairly different questions which both have highly subjective answers. You should do some thinking about the implications of a certain map pool and if its enough to keep players down until the next update/batch/whatever.
There’s never an upper limit to how many maps are appropriate for your game the same way there isn’t for how many maps is good enough. These are quantities you need to decide alone and weigh in what the gains or losses of each approach yields you. For example: too many maps may require players to memorise more and thus be less inclined to play, or too little maps makes the game repetitive and boring.
Like colbert said, there isn’t a limit to how many maps are appropriate for your game. Whichever amount you choose, it’s better to focus on adding replay value to those maps rather than constantly adding more onto the dogpile.