Is the approximate number of BCU-1525 in the market known at this moment?
There are 5000 BCU1525 in circulation.
Right now the most profitable public bitstream is Nexus, available on zetheron.com.
Please note that all zetheron bitstreams are available to all users, you just send a video of your hardware mining 0xBitcoin and you receive the non-public bitstreams.
There may be other developers that have truly 'secret' bitstreams that are not at all available to the average Joe. I don't know.