Quoting willoughfam:" Ah, don't I feel stupid. Thanks, I had no idea. :/"
I was clueless to that one as well. I went to motherhood maternity to buy bras and was shocked they had no DD's. The sales lady overheard me and filled me in on that one. It's kind of silly that they change it for nursing bras.
As for how much you will grow, you just never know. I have friends that only went up a size, I went from a C to a DD. I would get a sleeping bra for while your milk is coming in, then go shopping once your supply has regulated.
Best of luck with nursing! The first few weeks can be rough, but it is a very rewarding experience in the long run