How many shirts, pants, socks, shoes, etc for each family member?
How many toys for each child?
How many sets of dishes or towels or sheets?
For toys, DS has one bookshelf, a toy rack, and a toybox (for big stuff). We allow any toys that he plays with and that don't annoy the crap out of us into the toy rack. If it starts to get full? We purge. We also purge whenever anything new comes in.
Each family member has one dresser and one closet. That's it. Whatever fits in there is welcome, but no overflowing allowed (and no storing clothes for the winter/summer in a different place).
This has helped our family remain within reasonable boundaries without the stress of "ack! I now have 21 shirts!!" which didn't feel simple to us at all.
ETA: I guess simple for us means that our THINGS don't interfere with our life. We don't have too much, nor do we have too little. Either would interfere with the quality of our lives. We are a family that in order to simplify, moved into a bigger house with lots more land (we're homesteading on this land). It's more upkeep for us, but it allows us to be more independent, and to spend MORE time with our family on our land and less time as consumers in the commercial world. That, to us, is much more simple. Give me a day in the garden with all the work it entails as opposed to an hour rushing through our crowded local supermarket any day.