I have always been partial to A House in the Rift... pretty sure that counts as a harem game. Although, it has been a bit since I have gone back to it now that some updates have come out.
4-6 If the game has any story, to may more and everything gets watered down. If it is a game just meant to be a gallery of girls in scenes then it doesn't matter.
I actually enjoyed The Mist also but didn't quite consider it sandbox. It has been awhile but I remember it starting out a little slow. I really liked the art style though good scenes.
I think it can be a good preview of the quality scenes you can get. One of the best is imo was A House in the Rift which starts out with a nice scene but then you have to work for it afterwards. Was also kind of amusing