I'd say 5~6 LIs is definitely the sweet spot, and I wouldn't be bothered by a secret route. When it gets above 7 in a single game I'll start worrying about it lol
It also depends on playtime though. A longer game with more LIs is understandable, but a shorter one just tells me the routes won't be properly developed... as long as the plot is not rushed and the routes aren't the same thing, but with different characters, I can let it slide
It's more likely I'll play a game with a lot of LIs if they have two MCs and a longer playtime though. Too many LIs for a single MC is a bit overwhelming