In the live stream, Elon Musk mentioned that a dozen would be the ideal size for a mission. Could have easily been an off the cuff range, but would allow for some engineers if something goes wrong.
Haven't seen any official confirmation.
6 artists + Yusaku = 7 passengers so 5 engineers (or 4 + Elon)
8 artists + Yusaku = 9 passengers so 3 engineers (or 2 + Elon)