Short answer: Texas law sets no minimum employer contribution. If a carrier requires one (most ask for about 50% of the employee-only premium), it must apply that rule the same way to every small employer.
Unlike many states, Texas does not mandate a specific employer-contribution percentage. Under the Texas Insurance Code (Ch. 1501), an issuer may require an employer contribution “in accordance with its usual and customary practices,” but must apply that contribution level uniformly to every small employer it covers in the state. In practice most Texas carriers require the employer to pay at least 50% of the employee-only premium, and may also offer a plan option requiring a 100% employer contribution. The requirement is waived during the annual Nov 1 to Dec 15 open-enrollment window.