A VPS is just like your desktop. You can use the VPS chrome browser and go to your brokers website and download MT5. To run all 6 demo accounts at the same time you will need to install 6 copies of MT5. So to run 6 separate MT5 terminals, one for each demo account, you would need to order a VPS capable of running 6 MT5 terminals.
To run 20 robots in one MT5 terminal you would need to open 20 charts, one robot for one chart. MT5 can handle up to 100 charts.
Alan,