Is sleeping with 40 different men too many for a 23 year old girl? Advice Me Please

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My girlfriend and I have been dating a few months and we are 23 years old. I found a notebook the other day where she had written some private things. It it was a list of every single guy she had slept with. 40 guys total. This freaks me out a little bit and seems like a LOT of guys for such a young girl. I was actually pretty shocked. It makes me wonder if she has some kind of psychological issue or something to be sleeping with that many guys. She's been STD tested recently and doesn't have anything, so that's not the issue. For some reason I just feel like this is some kind of warning sign and just generally feel bad about it. Am I overreacting?

(yes I snooped and that is bad, but please ignore it for the purpose of the real issue of this question).

tl;dr: Is sleeping with 40 different men a huge red flag for a 23 year old girl.