42.
Jokes aside, I find it’s best to not worry about the number of maps, but number of key points in the game first. Establish your bare-minimum critical path (shortest, most direct path from beginning to end of the game), make a basic box map for each of those points, then make sure you can play from beginning to end with those basic maps.
Once that path is established, edit, improve, grow, and iterate on those maps as your game needs. If you need those maps to be larger, make them bigger! If you need to add more maps, add them! Let the needs of the game guide your development.
Another way of stating this advice can be found here: http://yanfly.moe/wp-content/uploads/2016/12/013.png
P.S. I highly recommend reading the complete comic that link comes from, it’s really solid advice for starting your game’s development in a way that will make the process more efficient: http://yanfly.moe/2017/01/07/comic-lets-make-a-game/
