Maneera, I wish I could say that my set number was developed according to a rigorous, well-thought-out mathematical formula, but the reality was it was number plucked out of the blue! I stood in front of my closet, looked at everything that was in there, and thought that 12 new items ought to be enough.
I really don't think the actual number means much in the grand scheme of things. After all, no one was going to come after me if I exceeded it. What I really wanted was a reasonable (to me) number that would make me pause before I slapped down my credit card.
I actually see this issue of "budgeting" from two perspectives. One is the monetary issue and the other is over buying. The monetary issue really wasn't a problem for me--I always had sufficient funds in my bank accounts to cover the cost of whatever I wanted to buy.
My problem was, if I saw something I liked, I'd find a way to convince myself the piece belonged in my closet. I did try to be a conscious, ethical shopper but taking inventory of my closet made me realize I just didn't need that many clothes to be well dressed. Too much choice was making me crave more, not less.
The real turning point came when I decided to drastically downsize my closet in preparation for a move to a condo in another city. I've so pleased with my smaller, but very functional, closet. The functional aspect came through loud and clear when I realized only a very small part of my former closet were 10s in terms of fit, flattery, and style; the rest of my wardrobe consisted of second-string duplicates, so-so experiments, lemming buys, emotional purchases, imaginary lifestyle clothes, items for occasions that never existed in my life, designer/label goggle buys, and "if one is good, why not buy 2 or 3 in different colors for variety" purchases.
For me, the number is just a fast and easy way to avoid those bad shopping decisions. I still make the occasional mistake, and I still buy with my heart, but thinking really hard before buying helps me figure out if the item will still give me the same surge of pleasure a few months from now.