My first City of Heroes server was Infinity. After a few months, I branched out to Virtue, and later Pinnacle. After that I cannot remember the order in which others were added. But over time I created characters on all the Engish-speaking servers. At mid-career I decided to have at least one hero and one villain on each. That way I could play "anywhere" should a friend want company.
I could end up with so many guys because players were allowed 12 character slots on each server. For several years an extra-character token was granted annually, allowing you to add one more character slot on one server. You could also buy extra character slots. Thus I eventually ended up with 24 guys on Infinity (my primary server) and 17 on Virtue (my secondary). I did not add any extra character slots to any of the other servers.
In 2011, Paragon Studios added a "free to play" (f2p) mode. I created a second account and created yet another two characters on Infinity. (Two total were all that an f2p player was allowed.) I also bought some more character slots for that account and added two guys to the Union server. With any real-money purchase, an account was upgraded from f2p to "premium" status. This allowed a few benefits to characters on the account, but still was far from the benefits granted to "VIP" (subscription-paying) players. The primary account remained VIP my entire career.
VIPs were granted a free character-transfer token every month. Plus there were a couple brief periods when Paragon Studios allowed unlimited character transfers. This is how some of the servers ended up with level 50 guys.