Unless we're talking nearly an acre of square gardening, the most we should expect gardens to produce is supplements to our pantry.
The sad reality is....without a continent wide logistics network hauling fuel, fertilizer, and food from rural to suburb to city...America could only sustain a tenth the number of people it currently does. If CT were 'cut off' by EMP or collapse of the dollar, Greenwich would have eaten all their dogs before a good harvest comes in to adequately meet the needs of the local population.
Now, fortunately, Connecticut has ALOT of wild life. When I lived in the New Haven/Orange/Hamden area I saw deer and wild turkey all over the place. Not enough to sustain the state's current population, but probably enough to sustain a town or two. Then you have the LI sound and open access to the sea....
But the rest of us...if the dollar goes into hyperinflation (as a result of various factors, not the least being the previous bankruptcy of major food producers in the period of deflation) and Obama's idiot savants ruin the private enterprise system leaving only corrupt and useless government logistics and lean but illegal black marketeers.... we will see hunger again. Hunger that'll be alleviated by back yard gardens - (at least we'll have more manpower what with millions of unemployed manufacturing workers), but hunger will not be entirely solved by them.
I have black berry bushes, a peach tree, 3 apples and 2 pear trees, and 2500 sq. feet of garden out back. I'm thinking of getting chickens, and learning how to trap wild game. But at the same time for $100 at Costco I can pick up enough staples to last my family about 1 or 2 months. That's hard to do with a garden even when you can it.
Still, if everyone had 2-4 months of food in their pantry and 1-2 months worth of basic edibles growing out back, (as well as a deer rifle, a shotgun, and 1000 rounds and shells....) we as a whole nation would be so much more robust and disaster proof. Sadly, America, like Rome, never prepared for the threat that's facing us right now with bared teeth.