You can have as many accounts on the same e-mail address as you want. I personally have seven alpha accounts on the same e-mail address no problem.
Of course I can’t nor would run more than one at the same time even if I could.
Though note that you can actually run several at the same time as long as you don’t actually enter the game beyond the character selection screen besides one at a time. So you can get your other account(s) ready for entry and once quit with your first one you can get in faster. You can also gather the daily login rewards this way while one of your Alphas is signed in without a problem.
Probably just a bug because unless the actual rules have changed it is against the EULA to run an Alpha at the same time as your Omega (or multiple Alphas) and you risk a ban or other disciplinary action against your accounts if you do so. Just a friendly warning to anyone considering exploiting the situation.