Alexander's eyes were drawn to the wall next to each of the girl's white flanks because on the walls were numerous black tally marks.
As for what they represented, well that was pretty easy to guess, for Alexander saw before anyone could start enjoying themselves, they had to pay to get a small piece of chalk which they could use to draw a line on the wall, thus helping keep track of the number each girl served.
And just going by eyeball numbers, Alexander already counted 30 for each of them.
It was ridiculous and represented a huge amount of income for the establishment.
As for the reason why the counting was done like this, well one of them was that it made double-checking the money collected with the men serviced easier, ensuring there was no one slipping in one a free meal, or that the cashier was not pocketing one or two pieces for themselves.