al_get_num_video_adapters man page
al_get_num_video_adapters — Allegro 5 API
#include <allegro5/allegro.h> int al_get_num_video_adapters(void)
Get the number of video "adapters" attached to the computer. Each video card attached to the computer counts as one or more adapters. An adapter is thus really a video port that can have a monitor connected to it.
On Windows, use al_set_new_display_flags(3) to switch between Direct3D and OpenGL backends, which will often have different adapters available.