How do I ask how many people he's been with?

I've been with my fiancee for a little over 10 months. Things have moved so fast between us, but what i'm asking has nothing to do with that. I'm confident in our relationship and I trust him completely. However, one of my friends said something about past relationships, which made me realise I don't know how many people he's been with before me. He was my first, and I'm really happy that I waited. I waited about 6 months into our relationship to first have sex with him, so I know he's really patient and he really cares about me. But this has been getting into my head quite a bit lately. When we first got together, we'd been out drinking with friends and we were on our way home, and he said that I was the most caring person he's ever been with, and he added (slightly drunkenly) that there was a lot. This didn't bother me at the time, but it has been more so lately. I just want to know how to ask him how many people he's been with before me, I just want a number, if he can give me one. I trust that he'll be honest, I need to know how to word it, so that I don't sound like i'm making a massive deal of it.