Hi, Welcome to JustAnswer! My name is Eric and I will endeavor to answer your question.
Most often the photographer keeps the negatives as his own property. This has traditionally been because they want you to have to come back to them for prints. With todays readily available technology however, you can do a high resolution scan of a quality photo and come out with 99.9% equal quality so it really no longer matters who has the negs with regard to the availability of reprints.
That said However, you can negotiate any contract you wish with the photographer. When we had a portfolio shot for my son years ago, we interviewed several photographers. We insisted that the negatives be released to us. One would not agree to that. Two or three said OK but it would cost more and the girl who actually wound up doing the shoot for us said no problem and agreed (In writing) to give us the negs and she did.
If you do not have a contract stating who keeps the negs and you are already done with the shoot, this gets into legal advice as you will probably have to sue. I am not verified in the legal cat and thus, cannot offer you my 2 cents on that. There are however, legal experts here who can advise you better on that.
If you are however, still just at the "shopping around for a photographer" stage, you can certainly insist that your retention of all negs is in the contract. DO NOT have this shoot done without a very well defined contract.