Determine width of rim to match tyre
Usually the inside width of the rim should be at least half the tire width and can be almost as wide as the tire itself. So for a 50-622 tire anything between 25 (a bit less would probably still be okay) and 50mm should work. 

The problem with wide tires in narrow rims is that the tire will be unstable in corners at low pressure and can break the rim at very high pressures. I’ve ridden 35mm cyclocross tires inflated to less than 2 bar on 13mm rims and  wouldn’t recommend it.