None, the supreme court ruled in Texas v White in 1869 that secession is unconstitutional and states do not have the right to unilaterally secede.

Under international law the right to self-determination has to be balanced against the right of the larger state to keep its borders and land intact. Judicial commentary has suggested that if the entity's right to internal self-determination, through the apparatus of the larger state, is thwarted then a right to external self-determination shall arise. So, although international law is unsettled on this contentious matter, the law currently points towards Texas being (legally, perhaps not in practice) allowed to leave the USA if its right to self-determination is thwarted by the USA as a whole (for example excluding Texans from the presidency or taking away their right to vote).