For example, a theoretical aircraft seats 154. It needs four cabin crew wether it has 1 passenger or 154 passengers.
If a crew member goes sick, you're limited to 150 passengers for a flight only to get you back to a crew base. The crew member then gets replaced or in exceptional circumstances, the four seats get turned into tables. The additional 4 passengers get offloaded. So you have 154 seats, 150 passengers and 3 cabin crew.
In normal ops its 1 per 50 seats. If your crew member goes sick and you're not at a crew base, then you can reduce the crew by 1 to get the ratio of 1 crew member per 50 passengers.