Sure, there's a limit on how many servers are worth purchasing. 3x servers might get us a 10% UX improvement, but that doesn't mean that 9x servers would yield

Sure, there's a limit on how many servers are worth purchasing. 3x servers might get us a 10% UX improvement, but that doesn't mean that 9x servers would yield a 20% improvement. We should stop buying servers when it stops being cost effective. That may or may not be 3x the number of servers we have now. It could be we're already over provisioned, or that you should only buy 2x, or maybe you can afford 5x, or 9x, and the number may change over time as the costs of maintenance and the number/value of your users rises and falls.