When would I have to file returns in more than one state?
If you're like most taxpayers, you'll only need to file one state return, usually in your resident state. You might not even have to file a state return at all if you live and work in a state that doesn't collect income tax.
But if you have income from multiple states, own out-of-state property or business interests that produce income, or your employer withheld taxes for the wrong state, you'll probably need to file a nonresident state return in addition to your resident state return. More info
And if you moved from one state to another, you'll likely have to file part-year returns in multiples states for that year. You might even have to file a third nonresident state return if you started working in the other state before you moved or continued working in your former state after the move. More info
TurboTax handles nonresident and part-year returns and will determine which state return(s) you need to file based on the information you provide in the Personal Info section.