As of 4/9/22, about 20-ish, I believe, out of an original team of ~70. The remainder either quit, were laid off, or were fired.

Source here, which expands further:

Before that’s a cause for alarm, however, many of these remaining 20 were key people in New Vegas’ development, and Obsidian also has Leonard Boyarsky and Tim Cain on board, even though both are on Outer Worlds (2), and Leonard and Tim are the original creators of Fallout.

Josh Sawyer, the director of New Vegas, is still there as well and has said he is open to working on Fallout again.