Poorly recalling from my memory. Given that a "block" is a variable metric, the actual number is less important than the concept that it is a static. There are only ~5 cities in the US with > 1,000 of these blocks: NYC, Boston, DC, SF, Chicago (Philadelphia?, Baltimore?)* AND with a transportation network that makes car-free living possible. Developers would build more such areas if zoning allowed.

* Forgive my ignorance for being from the West Coast. Even in SF its difficult to live car free if you don't live on BART or Muni Metro.