Supposedly there were 17,000 of them built. I have been attempting to assemble some information on surviving sets just to get an idea of how many are still around, so far I've heard of 30.
http://www.vintagetvsets.com/621database.htm
The serial numbers are ranging from 2400-15,000 so far, however I've noticed that most of the 630's that show up on eBay also have serial numbers that fall into that range, whatever that might mean?