Its commoners (that is, total population of all the realm's regions, not advies).
Well, part of it is so that realms that have more regions, but less population each, can be on more equal footing with realms that have less regions, but more population each. So, hypothetically, there could be a realm that does indeed have multiple cities, but they are smaller cities, comparative to a single-city realm (like say Ibladesh, Giask, Wudenkin et cetera). In which case, one would be more penalized by having more regions and one more penalized by having more population. Same as for regions with more gold than others, with realm economy adjustments.