rickman was a full on factory operation compared to seely, harris, dresda, etc.... between all lines they were selling 4000 bikes and kits a year at one point, that's frames and kits for 5 different 125cc engines for dirt/scrambles, road racers for triumph, norton, bsa, CR kits for kawasaki kz900, cb750, and kawasaki h1 (I only know of one in existence but there may be more hiding). Really they only had 3 or 4 designs (the 125cc kit would take a yamaha, montessa, etc...but if you bought a complete bike it was only powered by a zundapp). let's not forget the Royal Enfield rickman interceptors of which there were about 137 made.
I have always heard around 2000 of the CR units (750 and 900) and another 100 frames for the predator series of bikes. someone once told me that out of that 2000 units, 500 were kits - the rickman bros sold complete bikes as well up until 1976. I have no reason to believe these numbers to be accurate, it's just what I have heard. At a time when a cb750 was less than $1000, the rickman kit was $1400. it was a read deal premium special, given the price I am shocked they sold as many as they did.
It's odd, I have seen enough rickmans to think they aren't rare at all. there is a CR900 hiding at spannerland. I have owned mine for years now. I have seen the two that the AMA has at different times, I know three other people with them, and that isn't counting any I see on the web which has been quite a few.