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!