Different hospitals are different about the amount of clothes. Some are pretty strict I've heard. The first time I went I packed for a week b/c I had been told I'd be there that long. They were ok about it but didn't give me all my clothes, just a few days worth. I'm not exactly sure why.

I usually take a smallish suitcase and a backpack with my blanket and a pillow (I hate the hospital ones). They seem ok with that.