System Center 2012System Center 2012 Licensing

hi SCCM expert

Can some one explain how SCCM  licensing work simple way?

I want to use SCCM2012 to manage physical and virtual server infrastructure  as well and desktop computers

At this stage 4xphysicals serves
                   2xHypver V host with each running 6 VM's as cluster environment
                   45+ desktop computers
                  

Thanks