How many mongrels?

Here's a simple formula to follow:

A. Take the (average or median) request time, in seconds. Say, 0.250 seconds (250ms)

B. How many requests do you want to handle at peak? (e.g. 10,000 a minute, 166 a second)

C. Multiply A x B : 0.250 * 166 = 41.5

So you need about 40 mongrels to handle the load. At about 60MB per listener, that's 2.4GB of RAM, plus a bit of room for leakiness and swapping. Ezra at Engine Yard suggests "about 10 dogs per CPU core", which means that if we have a 4-core opteron box with 3GB of RAM, then this is possible on one box.

Your mileage will vary, which means, if the box is lagging, remove a few mongrels.