The effects of the sacrifice are going to change the amount of people who can safely be sacrificed. If they get mass healing, or can insure crop success, or whatever other magical mass mojo you can think of they aren't going to die as early or as often. You could even get a feedback loop where the population surges but requires ever greater sacrifices to keep it surging, which causes a kind of volatility as crime slows down and they either need to put mandatory maximum ages on people or invade their neighbors more frequently.