If you have a single PC which is strong enough to play all your clients, without straining its resources, I'd go with one PC.

If your single best PC cannot smoothly handle all the sessions you want to run on it, then multiple PC's is a good option.

You should be able to run at least two monitors on a single PC.
It depends on your available monitor sizes.
The main session should be large enough to see everything fine.
The slaves need to be large enough to notice if something is wrong.
With KC, the swap speed is not terribly fast, so the slaves have to be a bit larger then other options.

$20 per license is very little, but its worth mentioning.
You'd need one KC license per computer.