How much alcohol is left in this sauce? I make Rachael Ray's beef teriyaki with green beans, although I double the amount of sauce. I usually use dry sherry (rather than mirin), 15-20% alcohol. Given that the sauce is only cooked for a couple minutes, how much of the alcohol will burn off?
It is impossible to calculate. Alcohol is more volatile (in a chemistry sense volatility is a measurement of a substances tendency to vaporize) than water, but it doesn't disappear anywhere near as fast as many people think, especially when it's mixed in with water. 

You are adding the alcohol to a wide pan on high heat and stirring a lot, which is certainly a good combination for alcohol loss, but the finished dish will still have some alcohol in it.