So I think the biggest thing is deciding the demographics that get sacrificed. Also, I'm assuming you want there to be some kind of conflict around this? If so, what exactly do you want that conflict to look like?

Is there any tangible benefit to the families of the sacrificed? For example maybe a parent's sacrifice guarantees their children will be fed. Or maybe noble families must regularly produce a sacrifice to maintain their standing.

So in most cases I'd expect the elderly first.

Children might also make sense if people find themselves unable to support all their children.

Healthy young adults will require special explanation. Most of the cost of raising them has been spent and most of their potential remains.

You said "a few a month", so let's assume 3* 12 *70 = 2,520 people would be sacrificed over a full life.

For the elderly, let's assume half of people make it to 70 and it's just what you do at that age, so you'd need a population of about 5,000.

For infants, you could manage maybe 10% and still have population capable of growth. Examples aren't appropriate to discuss per forum rules. So 25,200 people, which is also about what you need for an ancient city state.

For young adults it depends on the economic system. I'd say maybe 2% in an agrarian society. So 126,000.