From JAR ops:
JAR-OPS 1.990 Number and composition of cabin crew
(See ACJ OPS 1.990)
(a) An operator shall not operate an aeroplane with a maximum approved passenger seating configuration of more than 19, when carrying one or more passengers, unless at least one cabin crew member is included in the crew for the purpose of performing duties, specified in the Operations Manual, in the interests of the safety of passengers.
(b) When complying with sub-paragraph (a) above, an operator shall ensure that the minimum number of cabin crew is the greater of:
(1) One cabin crew member for every 50, or fraction of 50, passenger seats installed on the same deck of the aeroplane;
My bold.