Calculate number of resources for a timetable

I'm trying to write code to estimate number of persons needs to achieve a job following a dedicated timetable and with constraints. Example : 42 sites, each person can work only 24 hours per week, 6 hours/day, 5 days/week. Each site opens at its own days. Slots from 6 hours/week to 18 hours/week max.

Any starting points would help. I'm looking for similar examples. Algorithm (pseudo-langugae) or python would be better.

I'm not a student. Just fed up to do this manually every year :)

Thanks a lot for any help