From my experience, it counts total prints being cut, meaning if you print one 16x20 it counts as one, and when you print a sheet of however many 5x7s or 8x10s it counts as one. Hope that makes sense. Sometimes I print a sheet of 2 or 3 11x14s, and it counts as one print.
I usually use the epson LFP accounting tool to keep track of prints and job counts.