If you've got lots of residential zones but not a lot of booster buildings then that will limit how much population you can achieve.
You'll need an average of 2.5m per region. If your average RZ population is 5000, you'll need 500 RZs. 10000 average needs 250 RZs, and so on.
With Specialisation buildings your should be able to get up to 15000 per RZ quite easily. That would require 167 RZs.
Take a look at how many RZs you have in each region, what the average population is, and work from there. And you can have some refunds highly concentrated and others much more sparse. 5m in a single region is possible, for example.