I usually say past is past. However, the full answer is… it depends.
If my partner told me that they had had relationships with men in the past, that wouldn’t necessarily be a problem for me, but I would want to be sure in my own mind that they weren’t (for example) gay and in denial… I mean, I would want to know that they were certain of their sexuality and comfortable with it, and that there wouldn’t be any barriers to our own relationship.
If my partner had children from a previous relationship, I would want to make sure that there wasn’t any sort of irresponsibility there. If there were 3 kids with 3 different mothers, I would want to know why, for example.
Likewise, if I was with someone who had been very promiscuous, I would want to understand whether this was a phase, how old they were when they were with most of these partners, and whether or not some of them overlapped (attitudes to monogamy, long term relationships, and how they viewed sexuality as a function of personhood).
Basically, there’s very little which would be an absolute dealbreaker for me, and I don’t especially enjoy quizzing partners and giving them the fifth degree (so I don’t). However, when things come out naturally during the course of a relationship, I think it does help to try to figure out the context, if you think there may be clues in their past which may affect your future. In this case, it’s not their “magic number” which is of interest, but the circumstances surrounding it.
PS I think the nephew in this scenario needs to deal with the insecurities which are making him hop from bed to bed, because if a partner said to me “oh, I slept with over 100 women, but it was because I worked for the military and felt like I needed to “keep up” “, that would be a huge red flag for me. It shows a profound insecurity and immaturity.