Dirty Little Secrets

So ladies here's the big one. Is it OK to not tell someone you are in a relationship with about a past partner? My number is low and sitting at 2. I'm ashamed and regretful of one of those though. It was a one night stand fueled by anger, bad choices and a little too much to drink. Do I tell them or is it OK to only say 1. I'm not sure what's the right thing to do so any advise would be great!