Beyond number of servers, another factor is rate of infrastructure growth. Bottom line is that investing in a provisioning & monitoring infrastructure early (say going from 50 to 100 nodes) would pay off in a 100 to 500 surge and beyond. My postulation is that, the flatter the growth curve, the longer you can avoid building custom infrastructure.

Sure, the argument can be made that growth rates are not a hard science, but one can improve that by monitoring growth trends and intangibles such as word-of-mouth, press & infosphere sentiment. Sometimes, one can just sense the impending detonation; and the sooner the investment is made, the faster the return and the smoother the experience. That’s assuming whatever is built actually works .

The other factor that must be taken seriously is that the anomalies one experiences @ 10/100/1000/10000/100000/1000000 node levels are different. I don’t think anybody beyond Yahoo/MS/Google have experienced all the circles of hell.