Ask and learn.
I never asked my ex, but there was a morning where we spoke about God knows what (the only thing I remember now is the number) and I thought I loved her enough and was secure enough to know when she offered the number. She knew that I'd only slept with three girls at the time- two one nighters and then her, so for all intents and purposes she was my first with any consistency. My low number for a 24 year old guy (She was 27 y/o) made me kind of insecure already, so when she told me I was her 25th partner the relationship never felt the same and eventually ended. There were many reasons beyond the number for why it ended, but it makes an easy symbol for it.
The stupid thing on my part is that I knew she'd slept around before we started dating, so I shouldn't have been surprised or so affected. It was great that she was easy when we were just casually fucking. When we started with the I love you's, the number started to haunt me. It made me jealous, overly comparative, and I started to resent her and treat her like the whore she was (number was evidence, in my mind). It really brought out some terrible things in me. But really, I brought out the worst in myself.
Numbers can reveal insecurities you never thought you had and part of me wishes I'd never known. But hey, I'm conscious of more things now so my advice to the OP - go ahead and ask (you've made up your mind to ask already) and be ready for some shit, most of it self-generated and propelled.