You can use multiple units of G900 to achieve what you want.
For 6 equal size images displayed in one monitor, user can use one G904 + one G901 to achieve it. G904 will receive 6 input sources and output two 3 equal images windows. These two outputs can be fed into one G901 and apply vertical POP function to get 6 equal size image output from one UHD output port in G901. After connect to the monitor, user will get 6 equal size image at the same time.
If user want to get 16 windows in one monitor, user need to use 5 units of G904. Each G904 will build one 4 window output. No. 1-4 G904 will build 4 separate outputs and each output with quad windows. After feed the outputs from No. 1-4 to another G904, user will get 16 windows for one monitor.