Determine width of rim to match tyre I need some 5 ray rims but the ones I find on the internet are way too expensive, so I decided to "roll my own" by creating a 3D model and having it machined from a piece of aluminum.

I've chosen to go with 20x2.125, or roughly 500mm in diameter and 50mm in width, tires for my rim. I'm looking at the nominal section widths of the standard and for this size tire I see that a size of 21C, 23C, 25C rim is required. Nowhere is mentioned what C stands for tho. I assume it's from crotched but I'd like a word of comfort on that. So basically this means that my rim's inner width (the one labeled 19 here) must be between 21 and 25 millimeters?
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.