Here I go, up to my usual pedantry.
Also note that Poitiers and Florence are busy trade hubs as well, and Constantinople, Rome and Thessaloniki will decline to as little as 30 000-50 000 so...
Another issue, very much related to this, is how densely populated the countryside is. A town needs to import almost all of its food, and pre-modern, that means you have less that 5% of population living in the city. Or rather, 5% is the high medieval estimate, eraly medieval is likely much lower. Which means that our busy trade hub of Florence has 1 000 people in the town and another 19 000 people in the villages around it - assuming a reasonable 500-1000 people per village, you get a total of 19-38 villages necessary for this city.
Poitiers with 9 000 people has rural population of 180 000, with 180-360 villages. Constantinople with 150 000 pop has a monstrous rural population of 3 million, needing 300 000 - 600 000 villages, or to put it in different terms, damn near entire Greece, to sustain it.
Also note that the absolute highest number of people that can farm land from a single town/village/point of origin is about 5000, provided they are surrounded by arable land. More people than that and you need to farm lands that is so far away from town you are better off founding a village there by virtue of travel time alone. That means that a city that has its farmers within its walls can have, at the absolute best, 250 city-dwellers and 5 000 farmers. That's... not really a city, maybe a market town? Or just a really big village.
The only way to fix these bottlenecks is to magic your agriculture into being way, way more productive - maybe that's what the sacrifices are for?
Provided you go that route, 1 farmer produces enough for 1+1/20 of a person. To figure out how many times you need to boost the yield to get to your city pop, take 5 000 as max farmers, 5 250 as max supported pop and multiply that 5 250. That means that should your magic just double the yields, your city can suddenly support 10 500 people on 5 000 farmers. Which is pretty neat, because you don't need crazy agrimagic, just making that corn twice the size will do.
Granted, most of those deaths are infants - compare average life expectancy at birth versus at 20, being 25 and 40 respectively - but still.
tl;dr If it is large enough to qualify as a city and has the corrseponding historical population, the 12 sacrifices per year are a drop in the bucket.