you can carry one (1) concealed gun at a time. you can carry as many as you want openly.
you qualify based on caliber, revolver and semi-auto (derringer too, but it's rarely done). you can carry up to the largest caliber you qual with.
ex: revolver, qual with 44mag. you can carry that caliber or any smaller in revolver, but you can't carry a 45colt (bigger cal).
semi-auto, qual with 45acp, same idea, so you could carry a 44automag (smaller cal, but more power), but not a 50cal desert eagle.
smallest cal you can qual with is 32 cal, but you can carry a 22LR once you qualify.
you can swap guns all you like, as long as you stay within your qualification boundaries. it's too your advantage to bring or borrow a big gun to qual day. after that, you can pick what suits your needs.