How dangerous are these cracking tiles if they contain asbestos?
Asbestos is only dangerous when it becomes airborne, which happens when  whatever contains it is turned into dust. For example it would be a very bad idea to remove these tiles by breaking them into bits with a rotary hammer, and even worse to cut them with a diamond disc grinder. The mortar/glue below may also contain asbestos fiber as reinforcement.

However in their current state, they are not dangerous, as the asbestos fibers won't become airborne.

If the building contains asbestos though, you might want to check the fireproofing and/or insulation. These are the most dangerous, as they are spray-on fibers and tend to degrade with time, releasing the asbestos into the air.