In an of itself, the number of partners is irrelevant. The who she slept with is also irrelevant. I mean, if you want to get into who they all were and how long their dicks are and how hot they were, you might as well forget it. Usually, this worry on the part of the man is a kind of competitive or insecure thing. She’s had a lot of experience. Will I match up?
There are things that would worry me more, and I do think it is relevant to learn about past relationships. How many of the guys were one night stands or just quick things? How long did the longer ones last? How did she meet them and why did she break up with them.
If there are a lot of one-night stands with guys she met that night at the bar, then I would be very concerned. Not about her morals, but about her mental health. As @Kardamom said, she could have been abused. A lot of people who have been hurt in childhood spend a good portion of their adulthoods “acting out.” It is a way of trying to feel better (it gives a high better than the high from any other drug) and to cover over feelings of low self-esteem.
Does she drink much? Does she like to go to bars? How did you meet her? Has she ever been in counseling? Do you know what for?
What you want to do is to understand more about her past, and then see if she is past it. If she is past it, then it doesn’t matter. If she is not past it, is she working on it? If she’s working on it and you can support her, then the past is not a problem.
From what you’ve said about her (precious little) it sounds like she is a different person. Perhaps motherhood has settled her down. Perhaps she has just matured for any number of reasons.
But you know what? These are things you’d want to know about any potential partner. You are involved with her and you have the information to make your assessment. Or you can get it. You probably have some interesting facts about your past that she’ll want to find out about. It goes both ways.
The number of past sex partners is not what is important here. It’s her past that is important and how she has run relationships in the past. It’s the preset and what she has learned from her past. I bet she has a lot of stories. I bet it will be fun learning about her. I think you’ll find that she’s been doing her homework, so to speak.