The Walton family, which founded the company more than five decades ago, owned 50.86 percent of Wal-Mart’s stock as of Dec. 31 through an entity called Walton Enterprises, L.L.C., according to Thomson Reuters data. In a statement, the company said it had been informed by the family that it planned to sell shares “from time to time” to ensure its stake does not increase as a result of the company’s stock repurchase program, and to finance investments in charity.