#8 refers to the size of the screw or nut. A #8 is .1640" dia and comes in either 32 or 36 TPI.
In a classroom setting it could also apply to the person sitting in seat # 8
Ah, I thought for sure I had the best answer!
As Re-design said, "#8" is a standard size (in the American "Unified Screw Series", most likely UNF).
It is the size of the hole and threads. It fits an #8 bolt or machine screw.