al_get_num_video_adapters - Allegro 5 API

       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(3alleg5) to switch between Direct3D and OpenGL  backends,  which
       will often have different adapters available.

       al_get_num_video_adapters - Allegro 5 API

al_get_monitor_info(3alleg5)

Allegro reference manual                                                      al_get_num_video_adapters(3alleg5)

              #include <allegro5/allegro.h>

              int al_get_num_video_adapters(void)