2 tax free.
I took 12 back last time I traveled. I'm an "elite" flyer on Continental so thankfully I don't pay for overweight bags. Other wise I would have never bothered. If you're going to lug it all the way back I recommend expending the effort to find some good bottles that aren't exported to the US because, as argsteve pointed out, most argentine wines are cheaper in the US than they are here.
Also a lot of bodegas these days are bottling their top shelf wines in what another expat once amusingly called Rolland bottles which are ridiculous monstrosities that weigh as much empty as a normal bottle full. If you grab a case of one of these wines you'll break your back for sure.
With respect to taxes, even though 1.5L is the technical limit on alcohol importation, I've never paid bringing in wine from argentina or europe. This is most likely because the taxes are so low that the customs officers just don't find it worth it. I remember looking it up last time I flew to the states it came out to be something like $12 for about $500 worth of wine.