All U.S. citizens are residents of at least one state for tax purposes. Your state of residence is determined by:
You can be considered a resident of multiple states. It’s also possible to be considered a full-year resident of one state and a nonresident of another state, or a part-year resident in multiple states and nonresident in other states at the same time.
If you don't know which state you should consider your state of residence, contact a state to understand the resident guidelines.