This question has been asked in more ways than there are Buddhas. Try looking here http://math.stackexchange.com/search?q=birthday+problemhttps://math.stackexchange.com/search?q=birthday+problem
birthday problem - expected number of collisionsbirthday problem - expected number of collisions <-how about this one, you should be able to find an answer from one of the answers of the linked problem.
I am not trying to be an arse; however, with one of the most widely known probability question of the existence of time, you have to check this website before posting.
trying to save you from being harshly downvoted.