<p>In ColdFusion, under Request Tuning in the administrator, how do I determine what is an optimal number (or at least a good guess) for the Maximum Number of Simultaneous Template Requests?</p>

<p>Environment:<br/>
CF8 Standard<br/>
IIS 6<br/>
Win2k3<br/>
SQL2k5 on a separate box</p>

The way of finding the right number of requests is load testing.  That is, measuring changes in throughput under load when you vary the request number.  Any significant change would require retesting. But I suspect most folks are going to baulk at that amount of work.
I think a good rule of thumb is about 8 threads per CPU (core).
In terms of efficiency, the lower the thread count (up to a point) the less swapping will be going on as the CPU processes your requests.  If your pages execute very quickly then a lower number of requests is optimal.
If you have longer running requests, and especially if you have requests that are waiting on third-parties (like a database) then increasing the number of working threads will improve your throughput.  That is, if your CPU is not tied up processing stuff you can afford to have more simultaneous requests working on the tasks at hand.
Although its a little bit dated, many of the principles on request tuning in Grant Straker's book on
CF Performance & Troubleshooting
would be worthwhile.