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.