I feel that Ashley and Val wouldn't share. The other girls, would, I guess. Maybe Rebecca wouldn't either... Dunno.
Yeah, that's what I thought too. Ashley wants to get together with you for a serious relationship, she would'nt want to be just another one of MC's chicks. And Valery from a story perspective, she's already had a hard time trusting people around her and judging by what her uncle did to her, she would'nt opt for a causal hook-up. Rebecca too is a shy girl with her insecurities. And if you played her route then on Chapter 2 just after the boutique event, she asks if you are serious about her, so I don't think she would be fine with sharing too.
From the other LI's I think Ms. Elizabeth, Aurora, Sung Ji and Vicky are also someone who will not be willing to share. Ms Elizabeth already had trouble in her previous school for being in a relationship with a student so I think she woul'dnt really go for being a part of harem. Same goes for Aurora, Sung Ji and Vicky, but we know very little from these 3 of them so they may as well become wild cards, who knows?
So, that only leaves Jade and Jenny who will be fairly open to the idea of harem, so what I think will happen for harem to be possible is something like this:- 2 main LI's will be in the harem route which are Jade and Jenny and then we can choose from the side girls like maybe Adam's fiance, Dani, Dani's streamer dormmate or the girl from Jenny's restaurant.