I'm pretty sure that this info (or rather, something akin to decent estimates) has been posted here sometime over the past four years or so.
Not trying to be glib, but I suspect that it varied a fair bit - maybe 20 or so at most for many of the titles. I haven't seen any evidence of proofs for titles like The Days Run Away or Post Office, Mockingbird, etc., but certainly they existed. On the other hand, I have a proof of Hollywood, and they show up from time to time, so there may have been more like 50 or so for that one. One of those in fine condition is probably not worth more than about $50-$75. The earlier proofs I've seen (Love is a Dog, for example) are usually listed around $300+.
Hopefully someone with better knowledge will chime in (and welcome back, CoM; you seem to flit in and out every 10 months or so). I'm just going by memory and what little I have of my own.