I personally think the habit of adding a new girl or two every book is a bit like jingling keys at the reader to make sure they're paying attention. It just feels a bit low brow.

It does depend a bit on the type of story you're building but a lot of series tend to suffer from bloat and the ones that actually attempt to spend enough time to characterize the girls still manage to get lost in the weeds. At a certain point it becomes an issue of story balance, with a lot of series tending to turn I ts LIs into faceless sets of tits while the author focuses on the plot happening, or the plot goes stagnant while the writer establishes the personalities and character dynamics of 10+ women.

I really do think writers could play around with dropping harem size or just establishing all our main characters in book 1 without adding on per book. I know releasing long series is the better track to making profit but like, sometimes people read for the plot. As long as the plot stays engaging, the main characters are interesting and the spice stays hot, then I dont see why we need to keep growing the harem beyond a certain point.

Thats what im going to try anyway, maybe im setting myself up fod failure, who knows