You can go for as many as you like, but the game length has to increase with it, so after about 10 you hit, like, an unreasonable length for an otome, unless you dont flesh them out enough.
I prefer games with 4 or 5 LIs.
If there are too many, it gets hard to play.
I think 4 or 5 is the perfect number to fully develop each character's route.