I'm pretty sure that this is never, ever, clarified in the books.
Measuring it by refresh seems okay, but which refresh would you use? The PCs level? Well now if you are submerged every bloke on the street gets 12 fate points? Man that's a lot of lucky people. The NPC's refresh rate? Now you've got to know it for every NPC... and honestly I don't. A subway crashes, and I've got 50 mortals at 2 refresh, 3 guys at 4 refresh, maybe one old guy at 6 refresh.... gurg..
So I just give people fate points according to story. I give every NPC a single fate point, just in case. If they are above goon level, then I give them two per session. Then I assign more points as the PCs give them fate points.