It used to matter back when there were dedicated PvP servers, but nowadays, open-world PvP is an option you can toggle on and, along with an older feature called cross-realm play that lets you play alongside characters from other servers, a good balance of Horde and Alliance players isn't as important.
Don't worry about the Alliance/Horde ratio of players on any given server if you're keen on PvPing. It used to matter back when there were dedicated PvP servers, but nowadays, open-world PvP is an option you can toggle on and, along with an older feature called cross-realm play that lets you play alongside characters from other servers, a good balance of Horde and Alliance players isn't as important.