Most of what I read said to plan on 1 cup and 1 band size increase with milk in, but since that's still a rule of thumb, for the hospital I decided to go just with sleep bras and tanks since they are S/M/L.
NB, there are different types of nursing bras! I found the partial drop cup kind (that has a second layer for your nipple area to pop out) drove me crazy. The second layer is meant for support I think, which I may just not really need as I've barely edged into a C so far. It was definitely worth trying a few on, the last thing you need is to be uncomfortable!
I noticed the last time I was at Target that they are carrying an Ingrid and Isabel-designed line. My I and I things are some of the most comfy I own.
Fwiw too, I did a BF class, and the instructor said that for a little while, most women find it much easier not to try and manage the special clothing since nursing is challenging enough on its own. So if you don't feel your wardrobe is complete yet, that may be perfectly fine for you!