My lot go with 1 CC per 50 or part, however on certain types, the number of exits drives the requirement for a higher number of CC than the 1 per 50 rule.
Infants? - we go for 10%, not included in the 1 per 50 calc. The max number if infants is limited (I think) by the CAA and practically on the day, by the number of infant restraint belts and also infant flotation devices carried on the aircraft - both of which amount to the 10% figure by a strange coincidence!