Tex. Code of Criminal Procedure § 36.29

(a) Not less than twelve jurors can render and return a verdict in a felony case. It must be concurred in by each juror and signed by the foreman. Except as provided in Subsection (b), however, after the trial of any felony case begins and a juror dies or, as determined by the judge, becomes disabled from sitting at any time before the charge of the court is read to the jury, the remainder of the jury shall have the power to render the verdict; but when the verdict shall be rendered by less than the whole number, it shall be signed by every member of the jury concurring in it.

(b) If alternate jurors have been selected in a capital case in which the state seeks the death penalty and a juror dies or becomes disabled from sitting at any time before the charge of the court is read to the jury, the alternate juror whose name was called first under Article 35.26 of this code shall replace the dead or disabled juror. Likewise, if another juror dies or becomes disabled from sitting before the charge of the court is read to the jury, the other alternate juror shall replace the second juror to die or become disabled.

(c) After the charge of the court is read to the jury, if a juror becomes so sick as to prevent the continuance of the juror's duty and an alternate juror is not available, or if any accident of circumstance occurs to prevent the jury from being kept together under circumstances under which the law or the instructions of the court requires that the jury be kept together, the jury shall be discharged, except that on agreement on the record by the defendant, the defendant's counsel, and the attorney representing the state 11 members of a jury may render a verdict and, if punishment is to be assessed by the jury, assess punishment. If a verdict is rendered by less than the whole number of the jury, each member of the jury shall sign the verdict.

(d) After the jury has rendered a verdict on the guilt or innocence of the defendant and, if applicable, the amount of punishment, the court shall discharge an alternate juror who has not replaced a juror.