You are not allowed to use multiple accounts to create an illusion of more than one person.
You may not create an alternate account simply to vote on a poll or back up an argument, mislead other members, or evade a ban. If you create a new account, you must not pretend to be a new member; this is called sock-puppeting. Note that this does not prohibit having multiple accounts (for yourself or individuals in your family) in general. Accounts used to sidestep bans will be suspended without question.
As long as you are not evading bans or pretending to be a completely different person, or the like, then it is fine to have an alt account.