Janice at The Vivienne Files just posted about this today. She suggests a calculation based on how many outfits you wear in an average day, how many items per outfit, how often you want to wear each piece you own, and for what time period you're calculating. One of her examples:
365 days
each piece worn 12 times
(here comes the math!!!!)
365 x 1.2 x 2 / 12 = 73 pieces of clothing
I think this is a decent place to start from, and is interesting per the recent French Wardrobe thread, as well as the conversations we had during the April 30x30 and the June challenge.
I included shoes in my count in April and June, and often have a topper/'third piece' most of the year except when it's really really hot, so my numbers would probably be 3.5 pieces per outfit. If I use the same numbers as she does otherwise, I end up with a total of 128 pieces - which is probably not too far off how many clothes I actually own, excluding outdoor coats and gear and such.