There aren't any population problems that I've seen, not outside the normal sparse mid-level zones that are normal for any MMO anyway. The new arena level for the current seasonal content is absolutely packed, because it's just non-stop farming of high level mobs and bosses in a relatively small space.
But that's not really relevant to a sales record, is it? I like the subscription-free model because I can pick it up again at any time and play as much or as little as I want. Depending on your standard of defining the term, you might have more 'active players' that way than with a monthly sub model.