Depends on what Campag headset you have. VeloBase.com can be of help on this. They will also inform you what size balls. About the number you need: fill the cups and take one or two out.
The one you mention look quite a lot like the later Nuovo Gran Sport, but the NGS uppercup has only 2 flats where the other use 6. Be sure you have the right one, as they require different balls I recall.