A rota shows who is there. It never shows who is no longer there — nor who is carrying twice too much.

Nobody counts this. And yet it is what runs out.
The question sounds abstract until the day the answer is five, and one of the five moves away. The number exists, and it works itself out.
Someone who served every month drops off the list, and nothing happens. Six weeks later they assume they were no longer needed.
Every volunteer spreadsheet ends up abandoned, for the same reason: someone has to keep it. This one fills itself — and it stops there.
A short list beats a broad promise. Here are both, without rounding.
Create an account, let the rota run for two months, and the screen will tell you what nobody had noticed.