Well, no players really know the populations on any of the servers and Gameforge is not telling us. That being said, they did announce a server merge that they are working on that is going to combine all the existing US servers into two. Reni and Artemis on one side, and the rest on the other. Not sure when this is going to happen though.
As for running into people, well a lot depends on the time of day. If you are wandering around in the early morning hours, well, you may see a few of the foreign residents and those people that never seem to leave the game. Busiest times seem to be the afternoons thru evenings.
I am on Govinda server, and there always appears to be a few players in Logar, standing around if nothing else, same for Varanas. I cannot speak for Reni as I have not visited there in a while.
The conventional wisdom has always been that Artemis is the most populated server. Followed by Govinda, Reni, Osha, Palenque, then the Pvp servers. But, as I stated earlier, they are not releasing population numbers. The other factor is alt characters. Since a person can have unlimited accounts, it may even be difficult for Gameforge to know how many active people there are on any given server.