If you serve these sushi as a starter, allow 4 or 5 per guest. For a sushi-party, about 10 and as varied as possible.
For sushi with raw fish, you should make sure that it's fresh and of the highest quality. You do not need a lot, so it's worth investing in something good - have confidence in your fishmonger.
It's also a good time to try lesser known fish: never use red tuna (Alabcore or Tunus, which will probably disappear from the oceans soon, as a result of human stupidity), but opt for organic salmon, mackerel or sardines.
Sushi can't be kept waiting too long, it's so impossible to make them the day before. 1 or 2 hours should be the maximum. The best way is to make them and eat them straight away.
For complete success, the best way is to start a sushi-factory: one person (or two) make little rice cakes, another adds wasabi, another adds fish, someone else looks after plates and their filling, and so one. This method allows fast production, and gets everybody, including guests, in the kitchen, talk and laughing, and you get nice fresh sushi.
It might be better if yet another person opens wine bottles and fills glasses (Robert is a master in this art).