Yea, at the time of the tests there was 1 server with 3000 (!) players on it when I was playing once.
Now it is one and a half year ago JC2MP got released officially and the players are all playing on different servers. Though since GTA V for PC is out, there's less JC2MP players. But I bet they'll all come back since GTA V gets boring after a while. Plus I heard R* has learned nothing of their previous releases, since Online is once again crap atm.