S.C. Code § 14-7-1940
After the grand jury venire has been duly qualified by the presiding judge, the clerk of court shall place the names of all qualified grand jurors in a container from which twelve grand jurors must be chosen. The clerk of court shall randomly draw twelve jurors from the container, and those twelve jurors drawn shall serve as grand jurors, together with those grand jurors selected as provided under Section 14-7-1910. The clerk of court shall randomly draw three or more additional jurors, with those three or more jurors serving as alternate grand jurors in the event one or more of the original grand jurors are incapacitated, excused, or disqualified during their term. The names of the alternate grand jurors must be kept separate and numbered in the order drawn and in this order, unless excused by the presiding judge, shall serve when necessary. The remainder of the grand jury venire may be discharged.
S.C. Code § 14-7-1940