Yes. IntentService uses a single worker thread.
Or to quote the docs exactly:
All requests are handled on a single worker thread -- they may take as long as necessary (and will not block the application's main loop), but only one request will be processed at a time.
From https://developer.android.com/reference/android/app/IntentService.html
This means that only one call to onStartCommand
can be happening on the IntentService at a time.