We did 2 registries and ended up putting a lot of items on both. We wanted to do a variety of items and prices and when we buy things leftover on the registry, we get a discount. I am also having 3 showers...one for family and friends where I grew up, one being thrown by friends where I currently live, and one at work. It's a lot, but we just scanned everything we needed and liked.
We originally had about 60 items on our registry and gradually added more... I believe to a total of about 100. We had two large showers (40 people at each) and about 65 items were bought off of our registries. The rest were other things people wanted to buy or things we wanted that they just didn't take off the registry, like books, clothes, shampoo, etc.
I used the registry as a shopping list more than anything else, so I put everything on it. Now I've got a coupon to buy anything I still need from the list at 15% off.