In our old house I had gates at the top of the stairs, and a gate on a hallway downstairs. The downstairs gate together with some other doors restricted the girls to the kitchen, dining room and family room, although once DD1 could reach the doorhandles we needed a gate across the doors as well. I think in a two storey house they're pretty much required equipment, and the layout of your house decides how many you need.

Our house now is single storey and I don't think I'll have a need for gates this time around, but that will depend on how adventurous this baby turns out to be.