The amount of layers that I apply usually depends on, not only the rocket, but the type of epoxy used.
LPR= 1 layer (2 part clear)
MPR= 2 layers (2 part clear)
HPR= 3 or more layers, depending on what kind of epoxy is used. JB Weld, PC-7, and any of the thicker epoxies is what I normally use...