How GRE score affect your acceptance rate
Maybe a little, but don't count on it putting you over the line. Admissions isn't a numbers game at most institutions. Lots of bits of evidence suggest that you will, or won't, be a good candidate and a success in the program. You need to build a complete picture that has enough good points to outweigh any bad ones. 

If you were a bad undergraduate that is a problem that you need to overcome, or at least explain. What were you bad at? Good? How does math fit in here, assuming that is your field? What else have you done? Do you have any research experience? What are your goals and do you have a plan to achieve them. 

All of these things can matter, but a good GRE is positive, at least. What else have you got?