Outside the farm, I think the Game will be calculate as per MAP.
Like
- Bus area
- Town + Community Center + Joja mart + etc.
- Beach
- Mountain + Quarry + Robin house +etc.
Because when I grew them up full in the fence next to CC, then I moved to backyard of JojaMart. It won't reproduce itself.
Not sure about exact number but I think it's around 50 maybe?
From my experience who try to got more of them in town.
After they got some enough number (yes with Male and Female).They'll try to flirt each other and gift birth a lil' one. But my past was, when you got enough number of them, They'll stop (naturally) produce any of baby slime. unless you put an egg to incubator that put down to area you want to grow them.