Well it depends how you define populated I guess. I see a bunch of servers with 90% AI (I don't even know why AI cars are listed in a multiplayer lobby). The actual number of humans online currently is 10-20. I assume at prime time on weekends it could be 50 or optimistically 100. iRacing has dozens of series running simultaneously with races every hour and furthermore, races are divided into splits depending on your skill level. I don't know what numbers you have been looking at, but even at the most inactive hours iRacing rookie Maza cup has around seven splits, each with 10-12 players, making it closer towards 100 players racing in just this category in every hour at minimum. At the more active hours, Mazda cup races have 20+ splits.