How many active Bluetooth devices can I reliably detect in a single space?
I cannot see that the Bluetooth Specification makes any reference to limits of devices in the same area. This is very difficult to practically determine as it is related to not only Bluetooth devices, but WiFi as well.

This paper takes a look at how the increase in device count, increases the time it takes for device discovery: http://www.shiratori.riec.tohoku.ac.jp/~deba/PAPER/Journal/WINET-onlineFinal.pdf

Also consider the following:


  Bluetooth devices work with the 2.4 GHz radio band, which is the same unlicensed frequency used by many other wireless devices. If many devices in the same area are all using the same stretch of bandwidth, it can lead to overall network problems, as the signals collide and information has to be resent. The Bluetooth signal was designed to change it's frequency many times per second to reduce this interference, but if enough devices are trying to use the same small stretch of bandwidth, interference is inevitable. Bluetooth 3.0, being used in devices created in 2010, uses the 6-9 GHz spectrum, so it will have much fewer interference issues. Bluetooth 3.0 devices are also capable of transmitting at 2.4 GHz to communicate with earlier Bluetooth technologies. Bluetooth 3.0, running at 2.4 GHz, has the same issues with interference as earlier Bluetooth technologies.


To summarize: You need to carefully consider the possible interference an time constraints that you will have. I suspect that 25 devices can be done. But to be sure you will need to practically test this