If it's slow-burn/well-written then the number will be higher. On the face of it LiL has a ridiculous numbers of LIs but the characters have all been given time to develop.
It depends, to make a good story and to develop it's character decently I feel it would be around 4-8 range.
If your objective is only to make a set game, the sky is the limit.
Played a game with 7 girls that you had to regularly divide your time for because their mood towards you over time, so if you didn't bang them they started to get upset with you.. Not the worst concept, but the decay was waaaay to fast. It turned into a pussy management game which took some of the fun out of it. I'd say Desert Stalker does it about right, where there are a couple principle LI's, but lots of other side options you can periodically hook up with. Then you have Harem Hotel where you spend so much of your day banging girls that you hardly have time to advance the story.
One of the best ways I've seen a dev deal with this was Inceton with lust theory. Banging all the girls with clones of MC. Can't remember the exact number, think it was 8. Starts as a Harem ends with an orgy. Also technically avoids that tag.... I think
In my opinion, having 4-5 women in a harem is normal, but more than 6 is too much. It also depends on the type of game. If it's set in the medieval era, more than 5 women can make sense. However, in a modern-era game, it doesn't.
I don't think there is a true upper limit, but there IS an upper limit on a 'core' group. I would say around 10 is the absolute maximum on this core group but quality would suffer with even that many. 3-6 is a better number for a core group.
There can be an endless number outside of this core group though. These don't need any deep character development and can be extremely 1 dimensional. They could even be one night stands that you never see again. This is a good route to go for creators who enjoy making interesting character models without expanding the character roster to the point where it gets clunky.
Another option is to have groups of women who are effectively 1 character. Like a group of three friends from school. These three girls are never interacted with separately and so they are effectively a single character to keep track of, but it allows for 3 different models.
But for the 'core' group it's best to stick with around 4-6 I think.
Probably around 4-5 would be my preferred maximum. Bigger harems tend to either have characters that are too alike, without unique personalities and charm, or they start becoming a bit extreme to differentiate themselves from the rest. I especially hate games that increase the number of characters by adding forced granny porn .
I think 5 to 7. 7 cause 1 for each day of the week but then that gets a little too much. 5 is better to keep them all unique, Then you can have group days.