That really depends on your app and your caching strategy. If you are doing something complex or memory intensive (or your app is poorly optimized), or if your users are highly engaged, it could be in the low tens of users! If you are doing a lot of caching you should be able to serve static pages on the order of several hundred or more per dyno.
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So I'm curious, how many users per dyno are needed for a typical CRUD rails app?
That really depends on your app and your caching strategy. If you are doing something complex or memory intensive (or your app is poorly optimized), or if your users are highly engaged, it could be in the low tens of users! If you are doing a lot of caching you should be able to serve static pages on the order of several hundred or more per dyno.
You can get a lot out of 1 dyno.
On two dynos you can have 1 do background jobs and 1 server content
obviously it varies, but you don't inherently need many.