For those wondering how they get the 'estimated' sinner population, there are more metrics for determining population than just a census, tax rolls, and so on. Just because they're SINless doesn't mean they don't have an impact on other things, especially when you start talking power, water, food, garbage, and so on. You can make some estimates on population density based on how things like that, and I'm sure the megas have a department assigned to refining those algorithms. Not for any altruistic reasons, natch, more like finding how many people they can sell to, and finding useful groups that no one will miss if they die in product testing. However, the numbers would be estimates, not exacts.
In the Facts at a Glance box in the new Metropole book, it lists the population as 188,736,519. That's a very exact number, so I'm inclined to agree with MM that this is the 'official' number, drawn from SIN records and other such things. The estimated SINless population is 8% on top of that. Makes the most sense to me.