I tend to leave my home 6 days a week, sometimes more and sometimes less.
An ideal week for me is one dress, one skirt, 4 dressy tops, 1 casual top, 2 pairs of trousers and 2 pairs of jeans. Jackets would be perhaps 2 times a week and cardigans another 3 times.
There are 52 weeks a year, so these are the number of opportunites per wear:
52 times in a dress
52 times in a skirt
104 times in trousers
104 times in jeans
208 times in dressy tops and sweaters
52 times in a casual top or sweater
104 times in jackets
156 times in cardigans
Now I can divide each category by the number of times I'd like to wear a particular item in a year and that is the number of items I should own per category.
If I go with 10 wears per item, these are the numbers:
21 dressy tops and sweaters
5 casual tops or sweaters
Please note, this doesn't count formal wear or lounge/gardening wear, but it's a great place to start for me. It doesn't take much of seasonal changes into account excepting that approximately 1x a week I would wear a sweater top and not need a cardigan, which is fair because in the summer I still wear cardigans for the AC and in the winter I wear sweaters often enough to offset no sweaters in the summer numbers-wise.
I'm also pleasantly surprised to find that I'm not too far from my estimates currently, and that makes me feel good. I need to do an inventory again after all this purging and with my new purchases to see exactly where I am.