To summarize everything mentioned above:
Currently there are still no AI traffic in TMP/Online, I believe it's because SCS software updates their AI system ever so often that it's difficult for the developers to implement such features onto multiplayer, maybe it will happen in the future? Certainly not for now and not anytime soon.
As for areas with many players on ETS2, places like Duisburg, Calais, Dusseldorf and surrounding areas are usually packed on the Simulation 1 server, depending on timezones as well - I've seen a constant 500-600 players on the server in the morning while reaching heights of 4000 players at night.
You should also avoid taking your World of Trucks (WoT) contracts into crowded places.