To increase the number of Video Wall displays you must to add additional Video Wall processors.
To Add a Video Wall Processor
1.Run the Desktop Client on the PC that should be added to the current Video Wall. Physical displays should be connected to this machine.
2.Right-click on desired Video Wall in the Resource Tree and choose Attach to Video Wall.
3.Repeat all steps described in "Configuring a Video Wall Display".
4.Switch to Video Wall Mode (see "Switching to Video Wall Mode").
5.Repeat the steps above on each Video Wall Processor.
Video Wall mode will be extended and will include displays connected to the newly attached Video Wall Processors.