According to MSDN documentation and Stephen Toub answer, my C# app should use every Logical Processor of every Processor Group because it is configured as required (see my App.config below).
I run my app on a windows server 2012 with a NUMA architecture: 2 x [cpu Xeon E5-2697 v3 at 14 cores each with Hyper Thread activated] => 2 x 14 x 2 = 56 Logical Processors.
My app start 80 threads either from "Thread Class" or "Parallel.For" and …