The question, "How Much Resveratrol is in Grapes" is pretty common but I'm not sure it's the right question.
Let's start with a different question, "Which grapes have the most resveratrol?".
Resveratrol is the "antibiotic" that some plants make when they're under attack. Whether it's disease or fungus, resveratrol helps the plant fight it.
Consequently the grapes that have the most are the ones that are
the ones grown organically and
the ones that are grown in northern climates
Australian, South African and Mediterranean grapes have less than New York, northern France and German.
If you're translating that into the wine you buy, buy northern wines - not southern. BUT there's one more rule when it comes to wine. Only red wine contains resveratrol. Since resveratrol is held in the skin, white wine doesn't have much because the skins are discarded early in the process. Red Wine uses the skins to help flavor the wine much longer - and thus has a lot.
Now, how much resveratrol is in grapes. . . That's an impossible question that some people will actually answer with numbers. Resveratrol is an antioxidant that is oxidized in the presence of oxygen. The longer resveratrol is exposed, the less potent and useful it becomes. Therefore, no grape has a consistent number, nor does it have a consistent number from one day to the next.
What you really want to know is how can you absorb the most resveratrol? Well the answer to that is red wine (except for some supplements like OPC Factor). Red Wine has some great things going for it.
It is sealed from oxygen before consumption.
Resveratrol is best absorbed in the mouth (known as buccal absorption). It just so happens that you hold red wine in your mouth longer than any other beverage before swallowing.