Short answer: All of them. USERRA applies to every public and private employer in the United States regardless of size, even an employer with only one employee.
Unlike COBRA (20 or more employees), USERRA has no size threshold. The Department of Labor regulation says it applies to all public and private employers in the United States, and gives an employer with only one employee as an example.
It also reaches foreign employers with a physical location or branch in the United States, for their U.S. employees, and U.S. companies operating abroad, unless compliance would violate the foreign country’s law.
For a small Texas employer, this matters most for health coverage: even a company too small for COBRA must let an employee on military leave continue the group health plan, for up to 24 months.