Thank you for writing in, as you already know you can only validate 1 production domain and 1 staging domain per license. I advise that you review the disadvantage of not validating the site (here) and see if you can tolerate that on your demo sites. If not, then you need to develop your demo site one at a time when finished revoke the demo site and re-assign it to next one until you finish all your demos. Else you need to purchase 1 license for each demo site (Envato standard licenses).
Of course, you need to validate your main domain as your Production site.
Hope this shed some lights