if you only need visual ui testing commands like VisualAssert or VisualSearch you can run many instances at once. Everything happens only inside the browser.
but if you also want to trigger actions with the real user commands xtype and xclick, then technically you can still run many instances, but we do not recommend it, since xclick/xtype take over the global mouse cursor and keyboard entry.
Note that Kantu XModules work also great under Linux, so you can the automation also in free Linux virtual machines e. g. VirtualBox.