if your ultimate goal is to become a potato, you can never have too many concussions. if you are trying NOT to be permanently brain damaged then the best way to go is to avoid getting hit in the head in general. there is no set number as to when permanent brain damage occurs, but the more concussions, the higher the chance of becoming a potato.
I think if you get one more good concusion then your doctor may tell you no more contact sports, a friend of mine was like you and his doctor told him that on his fourth one and he some how talked his doc. into letting him play