Google is your friend, from Mark Williams site:
According to the old SAE handbook, all modern axles are known as 24-pitch. If the shaft had a 1-inch circular pitch diameter (the mid-point between the major and minor diameters), it would exhibit exactly 24 splines (or teeth). The distance between the centerline of adjacent splines remains constant, so as the diameter of the shaft increases, so does spline count. For example, a 35-spline axle has a major diameter of 1.500 inches; a 40-spline axle is 1.708 inches in diameter.