A.H.
I had a little boy that was a serious wetter so we had a lot of changing and multiple washes (changed his crib 3 times one night!). Personally, I think under doing it is better than overdoing it. You can always enlist in the older kids to help with the washing. My son only lasted 2 weeks in newborn clothing (guess what he had most of?) and so picking up clothes along the way was actually easier and more cost effective because I knew how much I needed by then.
So for a real number, I would have at LEAST 3-4 for the day. You have day clothes, 2 extras for accidents, and pajamas. We also lived in the onesies and sleepers the first few months. I cried when I couldn't fit him in the zippered pajamas, so much easier than all those damn snaps in the middle of the night.