[Kpourdeilami]

You can start with one server with postgres installed on it and when you need to scale to multiple application servers, you can run postgres on another server.

For backing up postgres, all you have to do is setup a cron job that backs up the postgres' data directory to S3/Google Drive/Dropbox every hour/day.

If you want proper replication and failover then you can probably use 2 digital ocean droplets each for $5/month and another $5 VM for the application server itself.