You can never have too many...fill in the blank!
I was thinking of this as I folded DD's laundry today. Here are mine:
Changing table covers! As a FTM, I never realized I would need several changing table covers. Just yesterday we went through three between two pee episodes (guess the cool a/c makes her have to go in the middle of a changing) and a spit-up episode. It doesn't happen often but when it does I am so happy I have 4 to rotate!
Burp cloths/bibs! We go through these much more often than I thought we would. I actually had to buy a big pack of bibs when I thought for sure that I had received way too many as gifts.