I only bought one bra, one cup size bigger, somewhere around 12 weeks, and once I out grew most of my other bras I just cycled through 2 bras (the new one and one old stretched out one). Now, at 34 weeks, I just went and bought a nursing bra that is sized to fit a couple of band sizes and a few cup sizes. The lady at the baby shop helped fit me so that I had enough room to grow a band size and at least a cup size, and she suggested that I wait until a month or two of nursing to purchase any more nursing bras because sizes will fluctuate a lot until BF is routine. My new bra is Bravado brand, and I tried on a number of different styles till I found the perfect one (all Bravado though because of the easy-fit).
That being said, a friend of mine who is 12 weeks has already gone up almost 2 cup sizes, which is the max I've gone up in 34!