1 is for the SSR, 1 for the E-stop, and 1 for the controller permissive. I like to isolate the controller from the world with relays, optical or electromechanical. In general no direct connection between power components and the controller outputs or inputs. Normally the only thing I direct connect to a controller is the encoder inputs and the analog servo control outputs, everything else is isolated.
The exact relays that I use depends on the application, but when I'm doing my own stuff, normally whatever I have on the shelf.