I have my doubts as well regarding that. Here's hoping that you just need to make a bit of an extra effort with certain LIs, but still have a proper and complete harem if you do things right. I mean, you have abilities on the skill tree that specifically allow girls who don't get along to accept each other, so it would be weird to have that material and still limit how big the harem could be.
If I'm honest, I prefer to have exclusive relationships with each LI rather than a semi-harem. I mean, a harem is a harem, and the reason many of us like the genre is because it allows you to be with every single LI without having to 'sacrifice' others. If you end up having to choose or separate some others, then that's not a harem that I particularly like. If the original plan of 21 episodes still remains, then there's plenty of time there to properly address a full harem with the 18 main girls, but we'll have to see.
There's also the uncertainty on which 'role' each girl would have in the potential harem, because I can see Katie or Violet falling in love with the MC, but then you have the likes of Selina who just wants to fuck him and that's it. Here's hoping that there'll be more development with those girls that are more carefree and focused on sex.