Heroku's Production Check recommends "running at least 2 web dynos" for redundancy.
We're currently running one hobby dyno and are about to launch a social-networking app for iPhone.
The free & hobby dyno types only support a maximum of one dyno running per process type.
Do we even need redundancy at this stage?
If so, what's the most economical & redundant solution?
I personally wouldn't bother at this stage until you start getting significant traffic, at which point you would probably want to upgrade to standard dynos and start running more dynos anyway.
The metrics tab within the dashboard will have enough information for you to decide whether upgrading the dyno type or adding more is the right choice for the app.
Good luck with the launch!
It depends heavily on your application (how resource hungry is it, and how quickly does it boot?) and the nature of the launch (i.e. is it a soft launch, or is it to a large audience? Does uptime matter a lot, or a little?). Having monitoring in place (either with standard dynos and Heroku metrics, or with hobby dynos and using addons like New Relic or Librato) is essential.
For any production app when uptime is important, consider using standard dynos and preboot (https://devcenter.heroku.com/articles/preboot). This allows you to deploy patches without any downtime. Otherwise, make sure that your iOS app is set to handle maintenance mode gracefully!
