I've got five. I have a black and a white normal one, then the strapless ones I got were in a pack of 3 - a white, a red and a dark grey. I've been looking at getting a deep blue normal bra which I saw and liked but it wasn't available in my size last time I went in and I wasn't keen on the colours that were.
As for how long I wear them I tend to pick a bra to go with what I'm wearing instead of wearing a bra for a set amount of time before putting it in the wash basket and getting another. So I might be wearing my white bra one day, my black bra the next, followed by my white again, followed by my red strapless etc. I just check if they need washing when I take them off which can be after a day in the summer or after 3-4 days in the winter.