Imagine you build a computer, it's just about the same but with the addition of 1 extra PSU (You might not need it, depends on card model and PSU maximum wattage) and powered risers (Which are simple PCIe slot extensions).
Minimum required specs
60 GB SSD
4 GB RAM
Cheapest CPU (Usually around 50-70$)
So I will need to have 2 PSU? Why? Which GPU would you recommend and how many I can use in one rig?
It depends. If you use, for example: 5x RX 480 cards you will draw around 1000W from the wall (stock settings) meaning you might need a 1200+ PSU. I always prefer 2x 750 PSU when talking about this card models. Makes me feel safer.