Is it my any of my business?

ask him, first say hey I have a question that I have been wanting to ask you. I'm curious about your past girls? if you don't want to tell me that fine, I was just thinking about it. and let him choose if he wants to go into things. that's usually the best thing to do, give them a choice

Some guys are pretty open to telling a lot of their past, especially when
the girl has done so too. I, myself am open to my girlfriend asking me things.
I like telling her new things and answering questions. But I'm pretty emotional,
so anything she might ask might make me cry or something, but I don't mind.
I want her to get to know me more and stuff like that.

It depends on how your man feels about opening up. Some guys can open up without a problem, but some are a bit more difficult about it.

You should be able to ask him anything, since he's your boyfriend and all. If he shuts down and doesn't want to talk about it, then just move away from the subject and let him tell you about it on his own time. Prying doesn't help matters at all. But be sure to let him know that he can trust you and that he is welcome to tell you about anything.

we all have a past and a lot of like to leav our past in the past! so I would say just enjoy what you have and go forward not backwards if he ever wants to tell you then great but I personally wouldn't ask

Ask and be ready to either be upset about stuff or for him to be upset.