Start with 1, backup to S3 or EBS. Figure out how to overload that server. Once you have over 10,000 users start worrying about backups etc. If you server dies

Start with 1, backup to S3 or EBS. Figure out how to overload that server. Once you have over 10,000 users start worrying about backups etc. If you server dies and nukes all their data just start over no one will even notice. (Yes, your users may but if you have 10,000 users, its 10,000 out of 300 million accounts, just get new users)