if I have multiple versions (V10,v11,v12) on the same machine do I need 3 licences, one each, or is one licence (V12) OK.
A license for ifort is equally valid for older versions. Visual Studio integration won't accept so many versions in a single Visual Studio. You should for example be able to integrate 10.0 and 11 into VS2005 (with service packs) and 2 newer versions into a later Visual Studio, or have a version set up to run only from command line.
See also How can I use older versions of the compiler with different versions of Microsoft Visual Studio*?
Thanks for the info.