There is no fixed number of devices, because number of devices is a vague way to measure system resource use.
The RME docs *recommend* 5000 devices max, and for some features, less, see the RME online docs:
http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/cw2000/cw2000e/e_3_x/3_3/install/solaris/solinstl.htm#90835
These are recommendations, and your system performance will depend on the size of the server you plan to use. See the whitepaper on Ciscoworks in Large Scale Environments
http://www.cisco.com/warp/customer/cc/pd/wr2k/prodlit/ckspp_wp.htm
That will give you a lot of info on the factors that affect RME scaling. You should have a dedicated server for max devices, and you MUST follow the recommendations in the whitepaper for creating smaller views, minimizing syslog traffic, disabling availability monitoring, etc.