in europe its been 1:50 for as long as I can remember (10+ yrs). It used to be based on the number of seats installed but now it isn't.
Actually, under both the former JAR-OPS and current EU-OPS, the ratio is based on the number of seats. Here is the current regulatory reference:
OPS 1.990
Number and composition of cabin crew
(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 subparagraph (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; or
2. the number of cabin crew who actively participated in the aeroplane cabin during the relevant emergency evacuation demonstration, or who were assumed to have taken part in the relevant analysis, except that, if the maximum approved passenger seating configuration is less than the number evacuated during the demonstration by at least 50 seats, the number of cabin crew may be reduced by 1 for every whole multiple of 50 seats by which the maximum approved passenger seating configuration falls below the certificated maximum capacity.
COMMISSION REGULATION (EC) No 859/2008 of 20 August 2008 (.pdf)