There is no "1 exit for every 50 passengers" requirement in certifying aircraft for evacuation purposes.
They are bound by the distance between exits, the ability to evacuate all passengers with half the exits inoperative in 90 seconds and finally physical space, ie how many seats can you actually fit in the cabin?
The number of exits isn't really as important as their type, double width doors can obviouly allow a much higher number of passengers to evacuate than an overwing exit.
Types A/B/C are main cabin doors
(A is a double-wide, double-slide door, B is single-width, double-slide, C is single-width, single-slide)
Type I, II
or III are emergency exits
They range from Type I, found on the 757 at door 3, to type III, a standard overwing-exit.
Where loads are reduced due to inoperative exits, the MEL
for each aircraft type will determine how many passengers can leaglly be carried, dependant on the type/location of the inoperative exit.
There is a 1 cabin crew member for 50 seats (not passengers) requirement, but for some aircraft types there are further restrictions applied. For example British Airways' low-density 767-300s have only 193 seats, however the UK CAA have imposed a minimum crew complement of 7 on the type due to it's size. Obviously for a charter airline like MyTravel with 326 seats in the 767-300, the CAA and 1 cabin crew member for 50 seats rule give the same minimum crew complement anyway.
The maximum capacities for all UK-registered aircraft, along with a lot of other certification data, can be found on the CAA Aircraft Type Certificate Data Sheets