If the challenges reduce the number of jurors to less than twenty-four, if in the district court, or to less than twelve, if in the county court, the court shall order other jurors to be drawn from the wheel or from the central jury panel or summoned, as the practice may be in the particular county, and their names written upon the list instead of those set aside for cause. Such jurors so summoned may likewise be challenged for cause.
Tex. R. Civ. P. 231