At least 26 states would likely join them. There are succession movements in 15+ states. Strong opposition to the federal government in 26 states (the # that have filed lawsuits about the recent elections and federal laws - like zero border control).

It's likely that if Texas succeeded most of the midwest states would join them and literally nothing would be done about it. If the military was ordered to bomb civilians they'd likely all join the other side so no such orders would ever be given.

The country would split in two with the West Coast and East Coast being the only remaining part of the original U.S. and the entire Midwest being the new country. Almost all military veterans would likely move to the new midwest area and the the Coast country would be left with essentially nothing.