You may find different solutions, if you search on Google, now some of those won't work at all anymore or you'll be able to run only a few Skype instances at once, 2-3 at most. The trick i discovered is pretty simple: you need to run x amount of browser instances to be able to run x amount of Skype instances and you need to sign in to your Skype accounts via web: https://web.skype.com/ in each browser instances and click on the "Skype for Web (Beta)" button, so Skype will run in your browser, you still can run the Skype app natively too at the same time. I prefer to run the less possible programs and if Skype runs from the browser on its own tab, that's one less program to worry about and there's no need to jump back and forth between the browser and Skype app. I tested it with Portable Firefox instances and it worked like a charm. Simple, yet effective. That's all folks!