I'm never going to get down to 100 possessions, or even 500. I have more than 500 books!
But I do get something from reading about other people's attempts to limit their possessions. It makes me think about what I own and what I use and how much time/space/energy/money all these thinks take up in my life.
What I do find useful is counting a specific type of item. When I first started decluttering, it was the change of seasons and I was putting away my summer clothes and getting out the winter ones. I counted my sweaters. I admit, I'm a bit of a sweater addict. But no one *needs* 34 sweaters. Counting the sweaters made me realize that I had too many. I could wear a different one each day and not repeat one for over a month!
So I put all 34 sweaters out on my bed and sorted them, absolute keepers down to clearly give-aways. Then I decided that I would store the sweaters in two deep dressers drawers, and I started to put them away. After I put those sweaters that would fit into the drawers, I was down to 20. Still a lot, I'll admit, but I was ready to give away over a third of them, which I didn't think was too bad.
I have thick wool sweaters for being outside in the winter. I have dressier sweaters to wear to work, and casual sweaters for evenings and weekends. I could search and hunt for exactly three sweaters--one for outside, one for work, one for casual. But a) that would take a lot of time, b) I don't like shopping that much, and c) I don't want to deal with the comments if I wore the same sweater repeatedly during the week at work. And I enjoy having a bit of variety in the color and weight and warmth and style of my sweaters.
In 2010, I got rid of over 400 books, a bookcase, two area rugs, a TV cabinet, a rocking chair, three trash bags of clothes, 2 boxes of kitchen gadgets, and a box of general household "stuff." Part of my re-evaluation of these things came from reading about people who *do* count their belongings. Part of it came from reaching a point in my life where I'm making different decisions than I did 10 years ago, because a lot has changed in my life since then.
Counting possessions is a tool. Like any other tool, how useful it is depends on how you use it.