There is no built in limit to the number of slaves you may have. It wouldn't be too hard to implement a full 360 degree wrap around display using 6 computers and 6 projectors, each covering 60 degree field of view on a cylindrical projection screen. Pilots often look down and up, which increases the number of channels greatly for full hemispherical coverage.
Ideally, the master computer should be chosen to be whichever visual channel has the lightest graphical workload.
This might be the dedicated instrument panel. If the master computer has a heavy graphical workload, the other channels will usually lag one frame behind. Select the graphics realism parameters to ensure that all the visual channels consistently achieve a solid and consistent frame rate and, if your video card supports it, lock the buffer swaps to the vertical refresh.