I don't think you can do this if you build on the command line using devenv, but you can with msbuild. If you have existing scripts that call devenv then it's quite simple to change to msbuild:
Instead of:
devenv solution.sln /build Debug
Use:
msbuild solution.sln /t:Build /p:Configuration=Debug
You can then add the flag /m:N
to specify the maximum number of concurrent processes to use for the build (eg /m:4
)