Great question and great thread!
Right now i have 3, but i'd like to be more around 6. I find that the type of bra you wear, how well it fits, and how you treat them makes a big difference in how many you need and how long they last.
Two of my bras are nude, 3 piece stable cup underwires with side boning - this one here:
http://www.ebay.com/itm/FANTAS.....3cb1e98d99
i also have a black lace bra from the same company, the same as above except it's lower cut.
Stable fabric or lace cups last much longer than moulded or stretch lace cups. All bras last much longer if you give them a day to 'rest' between wearings and if you handwash and line/air dry them.My sister worked for years for a couple of Eastern European sisters who were trained in classical corsetry and had a lingerie shop in Berkeley. They said the worst thing every to do to a bra is to put it in a washer or dryer!!! The washer is hard on the elastic and lace (even in a lingerie bag, even in cold water) and your underwires will get bent and really mess up the fit.
So i've always handwashed my bras. It's not a big deal, just toss em in the sink with mild shampoo, real cold-process lye soap, or specialty wash like Forever New every few wears. Rinse and they dry overnite. It makes a huge difference in how they look and last, if you have budget considerations there is no other way to go IMO.
My current crop of bras (3) is almost one year old and they still fit perfectly and look great (even tho i've lost about 10 pounds since then). It comes down to buying a well built, well fitting bra and treating it with kid gloves. And i wear a 34E, so they do real work. Happy Sunday! steph