I think the OP's point is that these people weren't friends, they were people she was at school with perhaps 25 years ago.
I'm still friends with people I was at school with, and I'm about the same age as the OP's partner. I'm in touch with more of them, though I wouldn't say they're all friends, as we just haven't seen each other for years.
I think when you're at an event like a school reunion, there can be some tendency to revert to the age you were last all together. And I can see how that sort of conversation could come up, talking about people who were popular with the boys, and people who are now married, divorced and so on, and a few drinks in, it might get more personal - I wouldn't go into an event saying, "You'll never guess how many people I've slept with over the years!" but I can see how it would come up talking about relationships over the years (a natural conversation at a school reunion) and if someone asked, I'd probably just answer, without really thinking about it, because it's not something to hide, it's just not something you volunteer when it's not relevant.
I also don't think it's odd to know the number, because I have a statistical sort of mind, and it's the sort of thing I remember (among many, many other things.) I find it odd that people don't remember stuff like this, but I've learnt over the years it's just that we're all different.
I also grew up in a small community. People might talk about it for five minutes, but something else will soon be along, and it'll be forgotten about. And if they are going to talk, it'll be about her, not you, Pluto. If anyone looks down on you because of it, it's probably because they were already biased against you for some reason, and were just looking for a reason to confirm their views. There are people like that in life, but you don't have to spend any time worrying about them. And I think that most people aren't going to be that bothered. One of the good things about reaching my 40s is the realisation that mostly, people don't give a shit about what I do - I'm generally not even on their radar most of the time. Some of them might look down on me for a bit because of what I wear or don't wear, or the job I do or the house I live in, or the number of people I've slept with - but mostly, they're not going to care either way, because I'm not that important to them, and the people I am important to aren't going to be bothered about it. I've also learnt that the people you might expect to be really judgemental can sometimes be the most supportive and accepting of situations you find difficult, and it might not be the ones you think.
So I'm with a lot of other people on this thread - get over yourself, it's not that important, and you can't control who or what your OP tells things about herself to.