How many drinks is too many before driving?

I was wondering how much you can legally drink and then drive. Obviously having a DD is best, but there are many times when you just want to meet a friend or date for drinks after work. I have heard the rule of thumb is that it takes the body a hour to digest one drink. But what constitutes one drink? Surely a Coors light that has like 4 percent alcohol isn’t the same as having a double scotch. Those are two extreme examples, but some clarity would help. I don’t want to endanger society or get caught and get a DUI.