Yes, you can change IDEBuildOperationMaxNumberOfConcurrentCompileTasks by setting defaults write com.apple.dt.Xcode IDEBuildOperationMaxNumberOfConcurrentCompileTasks 14 or whatever, and this will effect all Xcode builds that start after that. Also, this will effect the command-line xcodebuild (but there, you can control it for each build with -jobs argument).

Did you specify inputPaths and outputPaths for your scripts?