Defines/Macros.

shader code stub

[numthreads(NUMX,NUMY,NUMZ)]
void CS(uint3 threadId: SV_DispatchThreadID)
{
    // do your thing
}

And then provide those defines through the shader compilation process. For the command line compiler it's -DNUMX=8 etc. For D3DCompile it's the pDefines parameter.

I occasionally grant defaults for such things, so I can check-compile without providing those values (again shader code)

#ifndef NUMX
#define NUMX 8
#endif

Edit: Don't know if you can get numthreads from a compiled shader easily through shader reflection, never tried. There's dcl_thread_group declaration in the assembly, but from a quick glance at the shader reflection interface it doesn't look like it's conveniently retrievable.

Edit2: Blind in the morning . See below, Thanks Tiago.

You can use ID3D11ShaderReflection::GetThreadGroupSize to get the thread group size of a compute shader in C++.

I do it in my engine right after creating the compute shader and store the thread group size together with the compute shader pointer in a struct, so I always have access to the info when I want the use the shader.