This is a measurement from left to right side of the winding area in a bobbin. If your humbucking bobbin has a traverse that measures .250 inches and you use .0026 inch diameter magnet wire, the maximum number of turns without the wire stacking would be on average 96 turns per layer. Divide .250 inch (traverse) by .0026 inch (magnet wire diameter) and you get 96 turns per layer.