Actually more typically my cities max out in the mid twenties, occasionally around thirty if I'm lucky.. The Alexandria case was interesting because I put so little effort into it. No changes (terraforming) in the region, no workers or settlers joining. In the same game most of my cities were around 18-25, though I did have one at 33 (plus or minus a little, fluctuating) at the time, I can't remember which. I did, however, manage to put about seven wonders in it, though I don't think this contributed much.
@Bartleby: 50? *boggle*