Put me in the "Caroline needs to have multiple exes for this to make sense"camp.
1) She has experience with a lot of different actions including the "Main Event" and knowing how to get a guy back up quickly. If she only had one ex, that might be reasonable, if that was a good, long relationship. However from her own testimony, her previous ex was bad. The desire to experiment and find new techniques may be present at the beginning of the relationship, but if the relationship is a poor one, that desire is reasonably going to drop quickly. As such, if her relationship with her ex was not good, then she likely wouldn't have such a repertoire of actions/knowledge. Multiple bad exes would've given her more opportunity to try out new things at the start of each relationship before it soured.
1a) As she's very self-confident and independent, she wouldn't have stayed with one bad ex long enough to learn all the stuff she knows, as well.
2) As the hopeful long-term choice for Caroline, it's better for MC if Caroline has a long history of bad boyfriends to make the one good boyfriend a treasure worth keeping despite the obvious drawback of being her brother. If she just had one ex, she could easily think, "One bad, One good. Dating brother is taboo, so let's try out there in the non-brother boyfriend pool again, since it's 50/50." A longer history of bad boyfriends makes the one good boyfriend she finds more desirable to keep in spite of the brother/sister taboo.
As an "oh, by the way", she's obviously not shy about sexual actions and posing in near nudity for her website, so she's clearly comfortable with herself and her body. Generally in a mid/late-twenties modern girl with that mindset, that equates to more than one ex in her life. I think reasons 1 and 2 are more compelling from the game's narrative to write her with multiple exes, though.
