Gonna depend on birth rate, of course, but also the natural death rate (which we will assume this is in addition to; they MIGHT be sacrificing the infirm, which would mitigate this a lot).

So, we're talking Natural Population Growth rate.

I'm not going to crunch the numbers from that site, but it looks to mostly be about 1-3% per year, so we'll say 2% per year. We will also assume that a combination of magic and technology makes these numbers more or less accurate to the population of our fictional setting.

So, you can manage 2 people, per hundred, per year, and keep a stable population. Population of 1000? 20 people. Population of 10,000? 200 people.

Now, you're probably going to want to be under that... I'd cut it down to 1 per 100, or even 1 per 500 or 1000, if you want the society to be stable or increasing in number. They might also use prisoners of war (no cost to their growth rate), but you might also see issues as their society starts expanding... is it enough to do the sacrifice in just the main city, or do you need to do these sacrifices at every major population center?

Lots to consider, but my verdict is: Probably about 1% of the population is a good maximum from a population growth standpoint. Likely going to want it smaller, though.