This is actually not an easy question to answer because there are quite a few variables here. Synergy, Multibox, Modified Synergy, Hardware KVMs, Hardware Multicasters, etc make it not very cut and dry.

It also depends on costs, your play style, desire for quick reaction times, your desk's physical layout, etc.

I would encourage you to read up about the various options and experiment around a bit then form your own opinion as to which setup works best for you.

In general, one computer, one screen and autohotkey is the cheapest way to go. It is also by default the least flexible and less scalable. Some of the flexibility can be re-added - but that requires scripting, etc.