Kate, where did 1:36 come from ?
It's 1-19 Not required
20-50 One
51-100 Two
Thereafter 1 extra CC per extra 50 seats.
Possibly you have mixed the statutory requirement (above) with what airlines actually do. Many operate well inside the legal limits. A decision questioned was Easyjet moving from 149 seat 737s to 156 seat A319s, which requires an extra CC member, up rom 3 to 4.