al_get_num_display_modes - Allegro 5 API

       Get  the  number  of  available fullscreen display modes for the current set of display parameters.  This
       will     use     the     values     set      with      al_set_new_display_refresh_rate(3alleg5),      and
       al_set_new_display_flags(3alleg5)  to  find  the  number  of  modes that match.  Settings the new display
       parameters to zero will give a list of all modes for the default driver.

       al_get_num_display_modes - Allegro 5 API

al_get_display_mode(3alleg5)

Allegro reference manual                                                       al_get_num_display_modes(3alleg5)

              #include <allegro5/allegro.h>

              int al_get_num_display_modes(void)