You can login to two servers from the same login account (the accounts are independent of servers). It's 100% ok to have just one account and use it to play separate characters on Blue, Red, and Green all at once ... though I wouldn't recommend it
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But also login accounts are free, so if you did need multiple it'd be no big deal.
P.S. But you can't move toons between accounts, so think carefully about which account you want your character on IF you plan to have 9+ characters on a server.