Basically, there is no limit, unless it's like there a side import/export business going on. (People do do that.)
But when faced with a €2 bottle of wine, what is a girl to do? Luckily, the U.S. allows at least one bottle of wine duty free, though travelers are able to bring back as much those fermented grapes they'd like, though additional bottles may be subject to taxes.
Generally, one liter of alcohol per person may be entered into the U.S. duty-free by travelers who are 21 or older, although travelers coming from the U.S. Virgin Islands or other Caribbean countries are entitled to more...
There is no federal limit on the amount of alcohol a traveler may import into the U.S. for personal use, however, large quantities might raise the suspicion that the importation is for commercial purposes... a general rule of thumb is that 1 case of alcohol is a personal use quantity.
So, the official limit is one liter, and the average wine bottle is 750 mL, so that means only one bottle of wine will pass muster when it comes to escaping duties.
In case you can't do the math and have been a little bit too drunk Googling, here's what that means:
Everything else may be subject to duty restrictions, but its really up to the customs officer at the station, and also one of those things where everyone does it and it's kinda not really a big deal.
There are some FAA rules depending on the alcohol content but because wine content is generally hovers around 12 to 15 percent, there are no restrictions on the amount that can be brought back. (Beverages that contain between 24 to 70 percent alcohol content are restricted to five liters checked.)
More or less, the restrictions have to do more with the duties than the prohibition of bringing back wine.
Obviously, there is no limit to the amount of wine bought at an airport.
Basically, what we are saying is that it's okay to bring back all that wine from Portugal, and that you should probably be more concerned it breaking inside a suitcase than anything else...