In the more stable chair conformation of the compound shown, bromine is axial. How many ring hydrogen atoms are anti-periplanar (dihedral angle 180 degrees) to the C-Br bond?

The cis isomer has the tert-butyl group equatorial and bromine axial. The only hydrogens anti-periplanar to an axial C-Br bond are the axial hydrogens on the two neighbouring ring carbons, C-2 and C-6, one on each. The count is 2, and these are the hydrogens removed in E2.