have you looked on here http://forum.milestonesys.com/yaf_postst4304_The-number-of-Maximum-Cameras-on-one-server-is.aspx
i'm not sure if it's the same architecture as yours but they suggest 200 is an upper limit, they also suggest it may be wise to spread the load over a few more servers.
I personally have near 300 cameras spread over 11 10TB servers - this more to spread the load and give better resilience more than anything