The movie “21” held the winning hand against the vintage-football comedy “Leatherheads” in the struggle for weekend supremacy at the box office. “21,” the card-counting blackjack tale distributed by Columbia Pictures and starring Jim Sturgess, right, hung onto the No. 1 ranking by grossing an estimated $15.1 million in its second week in release, according to the tracking company Media by Numbers. “Leatherheads” (Universal), directed by and starring George Clooney, pulled in $13.4 million in its debut, finishing just ahead of the $13.3 million for another newcomer, the comedy-adventure “Nim’s Island” (Fox), starring Abigail Breslin, Jodie Foster and Gerard Butler. Fourth place went to “Dr. Seuss’ Horton Hears a Who” (Fox) with $9.1 million, elevating its four-week total to $131 million. Finishing at No. 5 was the horror film “The Ruins” (Paramount), with $7.8 million on its opening weekend.