This gets asked a lot
You can totally run two or more keezers or kegerators or fermentation chambers with one RaspberryPi and a pair of USB-connected Unos or Leonardos. You can also layer on multiple packages, such as RaspberryPints for a tap list display, and have some fun using a passive IR motion sensor to wake up your tap list display when someone comes in range of your dispensing system - even play a greeting or music; or add a comprehensive temperature logger that you can get to from anywhere on the planet.
I'm running a configuration that would manage any two systems - kegerator, keezer, fermentation chamber. Pick any combination of two. Two instances of BrewPi running on a pair of Unos. At the same time, I have a five channel temperature logger, and a tap list display, that I can see from virtually anywhere (including the 19" display above my keezer - which wakes up from a black screen when someone comes within about 10 feet of my keezer).
Another HBTer is going to be controlling four fermenters using four instances of BrewPi, so there's plenty of headroom on the 'Pi side. The rest is connectivity - you quickly end up needing a USB hub to tie things together...