Harems are something that comes up alot.
Sometimes they're necessary E.G a highschool DXD fanfiction since it's common knowledge that harems are a thing there.
My advice for this is if your fanfiction absolutely must have a harem keep them small it doesn't matter how godly you are the only person who could possibly keep a 16+ harem happy sexually and emotionally is Naruto using liberal uses of shadow clones and even then its unlikely to work for more than a few months because of jealousy.
For Highschool dxd fanfiction a harem should be no larger than a complete peerage so 15/16.
For anything else probably no longer than 5 or 6.
Anything larger and girls tend to be forgotten about quite far in which can annoy fans of certain characters.