How many square to a case of board and batten siding?

It depends on the exposure. If you are lapping each board 1 1/4" you have a 6" exposure. This would give 6 square feet coverage per board, and would take 17 boards to cover 100 square eet (1 square)

Exposure (in feet) * Length = Square foot of coverage per board

1 square = 100 square feet