This has been an interesting discussion. I am in the camp of those who think there is no magic, across the board perfect wardrobe size. It all depends on your climate, lifestyle, and preferences. I live in a 4 season climate of extremes bordered by slightly different temperate periods, meaning that cross-seasonal overlap is quite low. I travel frequently. I also spend a fair bit of time walking outdoors in all seasons. In my professional life I fill multiple roles (writer, editor, professor, student) and I'm also a parent. I need a "dressy" outfit of some description at least once a month. I give public appearances as part of my author role, in a wide variety of settings.
What this means is that to function well, my wardrobe will probably be larger than that of a woman who lives in a stable temperate climate, does not work outside the home, and takes care of her kids (for example). And larger than that of a surgeon or nurse, who wears scrubs or a uniform most of the day.
For several years when I joined YLF, I had a very small wardrobe -- fewer than 70 pieces. There was a lot of turnover. I really couldn't keep many items for more than a year, aside from a few coats and toppers. I had to do laundry all the time, and I was woefully unprepared for many events. Even dressing for a very casual holiday party or "date" was impossible.
Gradually, as my income improved, I bought more and for the last few years my wardrobe has hovered around 130-160 pieces. That includes footwear and bags and coats but does not include undies, PJs, scarves, belts, or strictly gear items.
My ideal, I think, for this climate and this period of my life, is more or less in this range. This fits in my closet (with seasonal change outs to a spare closet) and satisfies my need for variety. I know what I have and I wear it -- if I don't wear an item in its season I seriously evaluate whether or not to keep it and generally let it go. I replace about as many items as I buy per year -- figuring on average wear lengths of 3-5 years, with coats and some dresses and toppers lasting much longer and some tops not lasting as long.
But for a very eclectic dresser, this might not be enough. And it might be too much for a very classic subdued dresser. Even if their weather/ lifestyle, etc. were identical to mine. So it is extremely personal!