how to modify the number of parallel compilation with Xcode

Question

How to change the number of parallel compilations when building a C++ project with XCode ?

I am not talking about distributed builds, but on simultaneous compilations of

Answer 1

I don't know why it drops from four to two, but I also found that quitting Xcode 6, then in Terminal entering "defaults write xcodebuild IDEBuildOperationMaxNumberOfConcurrentCompileTasks 4" got me back to four when Xcode is restarted.