I don’t have direct personal experience of the decawave system, we went straight to custom firmware and hardware. But I believe that is correct, you should be able to get range with just two and X/Y location with more.
The connection to the PC would be over a serial / COM port.
Check the documentation for PANS for more details on what the out of the box firmware can do (see this post: DWM1001 PANS Release 2)
Tracking lots of people/tags is possible as long as you don’t need particularly high update rates. As mentioned before if the out of the box firmware does what you need then that makes things a lot easier. If it doesn’t do what you need then you can definitely improve on it if you don’t mind making some trade offs.
e.g. our system gives a far higher position output rate (100 Hz), higher accuracy (2-3cm standard deviation), and the ability to switch seamlessly between GPS and UWB positioning. But we achieve all of that at the cost of number of simultaneous tags and flexibility (currently only 2, going up to 5 soon, they need to know about each other in advance).