The faceplate for my Atlas/Craftsman 6" lathe has four slots , as does my Tormach 6" RT. My Grizzly 602 8" faceplate and my Enco 12" RT have eight slots. Unlike a four jaw chuck which has some means of control for positioning, I don't think it matters much regarding alignment.
I suspect it has more to do with the remaining surface area. More slots mean less surface area so smaller faceplate and RT's would have fewer and larger ones more.