The top five states/provinces and the number of wineries:
There are about 450 wineries just in Sonoma County--more than all but the top three states!
Of the 7,762 U.S. wineries 1,197 are defined as virtual wineries. These have a label, but don't own vineyards or a production facility. They have a winemaker and use a winery belonging to someone else to make their product.
Another telling statistic has to do with the size of the wineries, measured by annual case production. Of the 7,762 in the U.S. 6,016 of those (78%) make less than 5,000 cases a year. So while you see wines from the big boys such as Beringer, Gallo and Kendall-Jackson you probably never see wines from the vast majority of producers.
That's a shame because you're missing out on some great wines. How can you get access to these? Make plans to visit a wine region this year. Thirteen states and two provinces have more than 100 wineries so there are plenty of choices!