1+2+3: To setup spacedesk you should have one primary machine and one (or more) secondary machine.
On primary machine you have to install the spacedesk Driver/Server.
On secondary machine, spacedesk Viewer app should be installed. If you have the 12 native monitors attached to it, then you can setup 12 viewer app instance on each monitors and connect it all to your primary machine.
You can easily setup the Videowall using the Video Wall Engine user interface.
For more details please check the spacedesk user manual https://www.spacedesk.net/user-manual/.
4. Current spacedesk BETA version is free to use until it expire on Dec.31, 2021.