in src/main/java/org/apache/maven/shared/invoker/MavenCommandLineBuilder.java [462:468]
protected void setThreads(InvocationRequest request, Commandline cli) {
String threads = request.getThreads();
if (threads != null && !threads.isEmpty()) {
cli.createArg().setValue("-T");
cli.createArg().setValue(threads);
}
}