Each physical or virtual server requires one FusionReactor license – all of the instances which are installed on that server are covered by the one FusionReactor license.

So, if you have (say) 6 physical boxes and 2 of those servers each have 3 virtual machines installed on them – then you need

4 (Physical) + 6 (Virtual) = 10 licenses
Virtual Machines (VM’s) are classed as separate physical servers and therefore require a license.