The legal limit in most of the US for drivers age 21+ is around one drink per hour for the average person… though not all of us are average.
The amount that will actually impair one’s driving, however, is a totally different ball of wax. I’ve seen people that could barely stand after half a bottle of light beer, and others that could function just fine even though they habitually got drunk enough to blow a BAC of .10 after eight hours of sleep.
Given that different types of alcohol “hit” differently, and people’s metabolisms vary considerably, plus the fact that some people can’t drive even sober, I would have to say that your best bet is to only drink when you know you won’t be driving anywhere.
Oh, and for those of you who have a few drinks and think you’re okay to drive, ask for a second opinion from a sober person. My rule of thumb is that if there is doubt then you’re probably too drunk to drive, and if there is no doubt then you are definitely better off in the passenger’s seat. I’ve pulled a lot of stupid shit in my younger days, but after being in the back seat of a car that flipped because the drunk in the driver’s seat wanted to hear the tires squeal around one more corner, I draw the line at DUI.
Sure, I’ve inconvenienced a few people by asking for rides or crashing on their couch, but I feel that those are both better than the alternatives; license suspension, prison time (possibly for vehicular manslaughter on top of the DUI), crippling, and/or death.