Generally speaking, most base stations can serve many (unlimited) receivers and are brand neutral, with the exception of Deere bases and recievers.  Deere can only talk to Deere as they use a non-standard form of communication.  There are ways around that with the newer Deere receivers but it isn't cheap.  If you have your own base station you can use it with as many receivers as you want.  You can allow others to use it as well.  If you are subscribing to a service it is up to them on how they charge you.  Per user or display or a flat rate.  I have a base station that serves 5 receivers.  I could serve 500 if I wanted to. They all connect automatically to the base when powered on and provided with cellular connectivity.  Some of them have built in cellular and others require being tethered to a hotspot or mobile device.  The connection can be as automatic or manual as you want it to be and mostly revolves around what form of connectivity you are using.

The biggest challenge is determining how you will get the service into additional displays.  Cellular NTRIP or 450/900 mhz radio are the most common.  Some displays have built NTRIP/VRS clients, others accept corrections via Serial NTRIP.  Some require additional unlocks.  Most have different wiring requirements and pinouts.  Of my 5 only two are the same.