I've run production sites with thousands of users on a $20 VPS, but it very much depends on your app and what it does. There is no simple answer to this. I thin

I've run production sites with thousands of users on a $20 VPS, but it very much depends on your app and what it does. There is no simple answer to this. I think the easiest way you can get some idea is a load test - ab for a simple test, and then perhaps httpperf or jmeter for simulated user tests. I wouldn't spend too long on it unless you are expecting very high traffic initially, but it can help you understand where your site bottlenecks are and what you will need to keep an eye on. Also, since you are running RoR, make sure you have New Relic up and running to get early warning of issues!