"the demons are targeting cities because they need many human lives"
Ok, sure, but why do you think that is all they would do? Have you not considered that they might simply stealthily be picking a couple people each month, or maybe week, slowly accumulating enough sacrificed lifeforce? Depending on how long they live, they could do 1 per year for 50k years. Or they could do almost 1 per day for a bit over 150 years. And if there are more of them, then that could be halved, or even further fractionalized. And maybe they simply do it until they are close, and then do a couple quick raids on some villages, for the last few thousands?
Besides, was there not proof of one demon being able to take humanoid shape? If that is the case, then they (and any extra demons with the same ability. All of them?) could easily just operate in some slums throughout several countries.
Hell, if they really want to go the long-con, they could kidnap humans and set up human farms. If they can pose as humans, that could literally be in the form of leaders of cities or even countries (even ones they otherwise govern really well) that they use to intermittently purge slums or "problematic" minorities (homeless? druggies? criminals? "criminals"? sick? "sick"? ethnicities? religions? and so on), or even wage wars or crusades where prisoners are ritualistically sacrificed for some "mock" religion (except it kinda is real).