TX can split into any number of states, as can any other state.
All it needs is for the state legislature and Congress to agree.
If any party had a sufficiently large margin in Congress and a number of states, it could split the each state it controlled into a large number of new states. This could in theory permanently pack both the House and the Senate. Perfectly constitutionally.
There were a number of proposals over the years to divide Texas but none of them got anywhere. One practical problem--how could all five states have the Alamo?