For each material slot you add to an actor, the mesh will be rendered again, so ideally you only want to add only as many materials as you have to.
You can use masks (often baked in your 3D modeling software) to create a texture with simple flat R G B (and yellow/magenta/cyan) that “paints” certain areas of your UV map. With the texture output, you can then multiply the appropriate color with whatever effect (i.e texture output) you want to apply to that specific area.
Look up at the default orange “unreal chair” material to see how it has been set up.