How many motors can be controlled using the Powered Up system?
With current hardware, the answer is 10. From the trains FAQ:


  You can run up to 10 trains at the same time from one remote control. There are 5 channels on the remote control (you can tell them apart by their different colors), and with each channel you can control 2 Smarthubs in separate trains with motors plugged in – as long as on one train the motor is in port A of the Smarthub, and on the other train, port B.


Each Smarthub provides two ports:



There are 5 channels available on current remotes, so you can pair up to 5 remotes with Smarthubs and control 5×2=10 outputs at the same time.