Here's the thing, the slides actually don't have a lot of preassure on them. So structurally you don't need that many, say one every 3-4'. But, and it's a big but, the more slides the easier they travel up and down the slot. Because of this ball bearing system like the Harken
Bat Car only need a point every 4'. While an old style slug needs far more say every 2'. A newer style plastic slide probably every 3'.
Of course it also depends on the length of the slides, material, mast
material, etc... The more the better except at the paying for it stage.