I'm going to let my gf know I only cheated once when it was actually multiple times
Why would you want to lie to her like that?
I'm afraid she'll leave me if she knows it was multiple times.
I'm not sure this information is going to be helpful to her. It is expected that you be loyal and faithful to your partner. She may not forgive you.
Why not?
Because you need to show your girlfriend the same respect that you want to see from her. Take the chance on telling her the truth, and hope that you can both move on with your relationship.