Based on DigiKey pricing for 20x TPS54292, that's AUD$162.9 for those chips alone.
Each chip has two independent regulators, so you can use one for every two RP nodes.
Use one regulator per RP node; don't try and "connect them in parallel".
Plus you're gonna need the 40x power inductors, a pile of caps and other components for your design, plus the fab of the 20 regulator boards.
Compared to a couple of PC power supplies, it's an expensive solution and high-risk. You might say that the cost is justified as a learning exercise, and you're free to spend that much money if you want to, but what are you really going to learn other than designing the components of a single regulator around the TPS54292?
If we assume, say, 80% efficiency, 5V out, 12V in, 1.5A load, the 12V input is going to need about 780mA current capability per regulator node, or say about 32A total.