I flew on a BA 321 , 2 hour flight with a club capacity of about 20 seats, they had 7 cabin attendants. 2 in club and 5 in economy.
In contrast I flew with LH on a 1 hour flight in a 737-300 2 class cabins both full and only 3 crew. They offered same service as BA and had time to spare.
A good on board manager is required (purser?)
I guess the more crew there are the more time they have to chat in the galleys with each other.