My girls can keep however much stuff they can maintain in a tidy manner. They don't have to keep everything spotless, but they have to vacuum and dust their rooms weekly. It didn't take long for them to realize that more stuff= more time spent cleaning and arranging things. :tongue_smilie: This made them willing to put stuff in bins in the garage. From there, it was easier to let things leave the house.
To me, 8 dolls, 30 outfits, and 70 ponies could potentially be okay. That is only three categories of stuff to put away. If that is what they spend their time playing with, I would encourage them to move the rest out of the clutter out of the play space. If they don't play with those things, then I would pack them up.
My girls play with only a few categories of toys, which makes clean up easy. (magna-tiles, Calico Critters, dress up, dolls, stuffed animals) They have large amounts of those things, but clean up is still fast, and all of it gets used.
This system works for my kids, but we know kids who spend a few minutes playing with every toy they own. That would be harder to deal with. :willy_nilly: