I used to think a harem shouldn't go over 5, but the isekai I'm reading now he's at 10 or 11 counting girls it's obvious like him and will join later, and they've all been introduced very naturally and tastefully and I don't have any complaints about the size of his harem.

Part of me feels like he needs to stop soon, and I still prefer 3P personally, but I can see 10 or so working now.