There's 2 types of overdoses. Fatal and non fatal. With benzos you're unlikely to reach a fatal state unless you mix stuff with your benzos. Specifically downers. If you're taking benzos alone you'll most likely black out and wake up 3 days later. And also a non fatal overdose is anything over the therapeutic level. Xanax is pretty hard to od on, mix it with alcohol and it's much more fatal. Oding on benzos isn't quick, and it's not particularly pleasant either. I would strongly suggest looking into another method. But if you're set on this method, you'd have to take a lot (adding alcohol, you won't need as much), but at least 50 mg I'd say. Also you'd want to make sure that you drink enough alcohol (but not too much or you'll just throw everything up).