How many USB monitors can be supported by a single PC?
A typical PC can run up to six monitors. The exact number depends on the type of content being displayed.
It would appear the Graphics card is no longer required if I am reading it correctly.
You need at least one monitor connected to a conventional VGA card (or integrated video).
Having said that what sort of quality does the mother board graphics produce?
The MB I intend using is i5 quality.
Better than any USB solution. Please not that starting with Intels "i" Series, the VGA subsystem is part of the CPU. Unfortunately (thanks to Intel marketing) not all i5 CPUs come with integrated video. And of course you need motherboard support also.
Some additional word about these USB video solutions:
These are combined hard- and software solutions. The more of these systems are attached, the more this will affect overall system performance.