That depends entirely on axle capabilities, who made it and when. Light weight RERs may have a 1/2" axle with one spline (keyslot) and a heavy GT may have a 1 1/8" axle with many splines. Plus there are at least a couple of spline configurations for each size of axle. Axle diameters go up in increments of 1/8" as a rule.
For instance, Chevys came with 2 different splines on the same size transmission tailshafts back in the 60s, and I think on the axles as well.
There is no easy answer without specific information.