There were about 12K at UVA last year (that's right, 1/5 of the crowd...and closer to 1/4 of those present after the weather delay); closer to 15-18K at ND in 2012 (FWIW the ND fans we sat by had made every ND game for 20+ years and said it was the best showing by a visiting team they had seen). A fourth of the GT crowd in 2012 was BYU fans. Half of MTSU last year was BYU. Ole Miss was middle of nowhere and it had ~7K, FSU had more than that.

There are a boat load of BYU fans that travel to road games. At least in the South. When BYU doesn't play near you every couple years, you don't mind the drive. Even if as many as 10% of those in attendance at LES make a long drive (4+ hrs) to LES, that's only about 6K people. Every road game I've been to for BYU in the past 5 years has had more than that in BYU fans.