You shall not operate more than one Warzone account that participates in the same game, tournament, ladder, clan war, or in any way gives you an advantage or gives you points or coins.
You also can't use alts to bypass QM voting limits. This rule is not enforced, as far as I know, but the Santa Claus exception above might apply. For ladders, you also can't have overlapping ladder runs. You have to wait for the games on one account to fully expire before joining the ladder with another. For Clan Wars, you can't have two accounts compete during the same season.
It's acceptable to have multiple accounts on the same team.
Unlike other online Risk-like turn-based strategy games, Warzone is incredibly tolerant of having and using alternate accounts so long as you don't use them to bypass limits or undermine the competitive integrity of the game. Be warned: if you undermine the competitive integrity of the game, you might find yourself one day bearing the horrific torture of running Clan League, so tread lightly!