The black squares appear randomly. When I did mine, I was counting all the grey squares, and think the lowest number between them was around 8 or 9, with the highest being into the high 40’s or early 50’s, so quite varied with numbers in between.
Was one of three completing the test, and what we all picked up on was that I’d started first. Because you hit a certain button, what you could hear was me pressing it, two seconds later the guy next to me, then the guy across the room a second later. Gave us all the confidence that we were getting it right....or wrong!!