is there a ALLEGRO_MOUSE_MAX constant I can use for reading inputs?
there is a ALLEGRO_KEY_MAX so it would only make sense.

There will never be more than 16 buttons on a 'mouse', ever. But you can call this function :
https://liballeg.org/a5docs/trunk/mouse.html#al_get_mouse_num_buttons

16 buttons... next they put a motiontracker under your keyboard...

I can't imagine a 16-button mouse, it must be a damn monster...

Logitech G600

ah ok.
why does allegro use a function rather then a Constant?

If it was a constant, it could never change, and it wouldn't be accurate