Then at dinner last night (for ten) we got to talking about this very question and it turns out not everyone agrees with me about the number six being ideal. Cure-taker klt108 says she used to regularly "host regular dinner parties for ten in my tiny East Village studio." I'm impressed.
Thinking about how many to invite to dinner is an important part of planning a meal. With too many, conversations get broken up. For a festive mood, I find four too intimate. That's why I vote for six. It's not to say I never have more or less, but when six sit down around my table, I find the balance is usually just right.
If you like ten, give us some details. Have another luck dinner party number? Do tell!