Guess-estimate from some dealers. Maybe more, maybe less. Who knows. This is after all a book from 1962, not 1939.
There are, to date, 1743 Universal, 39 Qualified and 276 Signature copies. Roughly 2000+ copies. And that number is certainly growing. Some will surely defend that CGC census numbers are flawed because of resubs, etc. I am leaving cbcs and pgx copies out of the equation, so it should balance things out.
GAtor mentioned an individual that alone held over 60 raw copies (in nice condition) iirc. Like him, I'm sure there are many others that hoarded them over the years.