I guess we only ever really NEED just one bag at a time, but it really depends on your lifestyle.
Some women can get away with one bag, and just buy another when the other gets worn out or they grow tired of it.
Others like to have maybe a smaller, nicer bag, and then a larger everyday one.
I haven't been using bags lately, but I like to have a small variety...an everyday bag that can take a lot of wear but it also comfortable to use, a nice professional looking bag for job interviews. I also have some really comfortable bags in different colors that I like for different seasons. I guess I couldn't get away with having just one at a time, because I do like to switch it up every now and then. However, I no longer feel the "need" to buy a new one every month.
I think my point (if I get to it, haha!!) is that everyone needs their version of a well-rounded collection. I have my nice leather bags (pink, purple, blue and a classic black), my comfortable cotton bags (yellow, black, white, brown) and a few larger totes for when I need my computer or some extra clothes or whatever. Maybe someone else could just get away with one brown bag and one black bag, but I prefer to have some color since I wear a lot of neutrals.