Some RZ guys still believe that there are 22k units in operation.
Sorry everytime any RZ unit is rezzed that number goes up one. So when somebody takes theirs down to put it someplace else, the number goes up. Whenever something goes wrong with it and they take it down and rerez it, the number goes up (rerezzing is a common fix after updates from a few things I've read). Lets not forget about taking it down so you can put a stealthed one up (which still counts to the system as putting a unit up).
Also notice the number went up this last time when it was pulled from the Marketplace and Soft Linden himself went down and took down the vendors. So no new units were being sold... and the number went up. Funny how that happens isn't it......
Also I bet its pretty likely that whenever somebody sets up the demo version the number goes up.
So how many are there? Well considering a lot of places have also taken theirs down completely, that some aren't used for scanning and were bought by stalkers/griefers for the HUD and that some were also bought just by people to see how they work.
I would put that number way WAY lower than 22k.