Upon importing multiple files via the Finder app (and its extended libraries), CoreServices will decide on how to handle NSApplicationDelegate's application(_ application: NSApplication, open urls: [URL]) method.
This generally results in CoreServices making two separate calls to the method – the second occuring roughly 0.05-0.09s proceeding the first.
Until Apple fixes this issue, a file queue can be created, preferably by implementing an input-buffer on the main thread. There is no way to predetermine the number of calls that CoreServices may make to the application.