This is kind of a weird question, but the good thing about forums like these is that they usually have people with weird knowledge.

Anyway, I have an idea involving a city that regularly executes/sacrifices people for important and magical reasons. What I'm wondering is how big of a city it has to be for the sacrifices (I'm thinking maybe a few every month or so) not to have too much of an impact on the population. Basically I'm wondering what the smallest sustainable population would be for this to work long term.

The tech level is approximately early medieval, with some magic (but mainly for the upper class so it probably won't affect the population numbers very much) and the city is very isolated so the effect of immigration/migration on the population numbers should be pretty negligible.