Female American Pitbull in Heat with Male

Question
My female American Pitbull is in heat and she and my male pitbull have "locked up" seven times that I or one of my family member have seen.  Like today for instance, they locked up three times which would be the 5th, 6th and 7th time that we have seen them in such an act.  I was wondering if this was a natural thing for them to do it that many time.  Sorry, I am not very good with canine terminology but by locking up I mean my male dog penetrated my female dog.

Answer
Well, if you leave two dogs to their own devises, then they will breed as many times as they can during the right time of a female dog's heat cycle. However, it is not advised to let this happen. I would advise to keep them apart for now. Good luck!:-)