from the PSU typically one (on hotswap supplies); e.g. on https://cdn11.bigcommerce.com/s-qfzamxn9kz/images/stencil/or... the connectors on the left are just control/sense pins and the big beefy ones are a single 12v & ground.

They plug in via an edge connector straight into the mother board, or sometimes into a separate board that connects to the mother board.