Not many. But everyone knows that Jimmer is a member because he played for BYU. Same with Steve Young, Ty Detmer, Shawn Bradley, and Danny Ainge.
Did the national press or public mention that Haloti Ngata was a member when he played at Oregon? In the NFL? Not really.
Even few members know that some great baseball players were members: Harmon Killebrew, Roy Halladay, Dennis Eckersley, Jack Morris, Jeff Kent, and so forth.
When a member athlete plays a high-profile sport for BYU, their Church membership is part of their sports identity and becomes part of the story told in the media and consumed by the public. When a member athlete plays elsewhere, that doesn't happen in the same way.