A person's "sex number" isn't relevant for jealousy reasons. Of course a person in his/her 20s or 30s is going to have had sex before, and as long as they aren't historically cheaters, the fact that they had relationships in the past isn't some kind of deal-breaker.
Where the dreaded number becomes relevant is that it tells you about the kind of person you're dealing with and whether you and (s)he are on the same page when it comes to love, sex, and relationships.
Some people consider sex to be something important. Something intimate and special, done between people who are in love, as an expression of that love. These people will tend to have a low number, and the entirety of that number will have occurred within the context of committed monogamous relationships of a respectable length. People that (s)he loved.
Other people think that sex is a fairly trivial, fun, primarily recreational act. Just something people do to blow off steam. Sure, they've had past relationships that were, of course, sexual. And they've probably also had one-night stands, some friend-with-benefit/fuck-buddy relationships in between real relationships, and so on. And their number is impressively huge, quite a bit of it outside the context of conventional relationships.
If your partner's number is huge, that's an indicator that your partner doesn't consider sex to be something extremely intimate and special. More like a video game. Something trivial that people do for fun. A mere physical act. If you don't feel the same way about sex, you're probably not a good fit for that partner.
This is a very well thought out comment and adds value to the conversation.
I think you have articulated what I had in my mind. If my wife knows how large my number is, and I have reason to think hers is in the single digits, how is she going to think about our relationship. Is she going to think of it differently? I don't think there is much to gain by letting my number out, so I don't - to maintain the status quo.
Do you really want to conceal who you are, what you're like, and what sex means to you...from your wife?
I've told my wife about my past, just not the exact number.
Most high-number people rationalize that by claiming that sex is completely contextual. It means one thing in one circumstance and one thing in another circumstance. Therefore, the exact number doesn't matter.
A guy or girl like this will focus on the fact that (s)he has never cheated on someone, or cheated only rarely when much younger, and even though (s)he has had casual sex outside of a committed relationship, it was still meaningful with a person (s)he was very close with. And even though (s)he has had (potentially numerous) one-night-stands, those don't matter so much because they were "just sex", and (s)he never saw those people again.
But today (s)he is with you, and sex means something every single time, because sex can mean different things at different times and places with different people -- so the number doesn't matter! No judging!
Usually, this situation happens in reverse, with the girl being the formerly promiscuous one. Married guys who have very tame, once-a-month and birthdays-only sex with a very sexually reserved and conservative wife, learning that, holy cow, she was a major freak in college. He had to take her out on 24 dates just to get to that first time, but three years before they met, she was screwing strangers in bar bathrooms in positions she won't try with him because she's not like that any more and it would make her feel slutty.
I don't read about the reverse as much, where the guy has the more promiscuous past and the girl's the one freaking out. Women tend to be more likely to freak out if you bought your last girlfriend flowers every week and took her on European vacations, but you took your wife to McDonalds on her last birthday. The sex isn't as big of a deal for them.
@archwinger we reached max depth on comment replies.
You have VERY informative comments, which leads me to believe that you are trained in these matters.
Maybe I should tell her my number when she asks next. From what you say, I don't think she will freak out - but she will probably probe further. I will have to think on it. Again, thank you for taking the time to analyze and reply.