I'm starting a service that offers websites that runs on a giant Wordpress multi-site. I have a cloud based vps with nginx-php5-php-fpm-mysql. Storage with Amazon S3, distribution with MaxCDN. WP Total cache takes care of this for WP. I do expect to be busy from the start as far as the number of sites created. My question is about mysql replication.
Should I go ahead and create two more VPS's now, and have one as master and one as slave from the get go? Or, is this something that can be done when it's time to scale? I worry about the size of the db and incomplete backups.
With, currently no service launched and 0 signups, should I even be worrying about this? What # of sites created would justify scaling wordpress?
I want to start this off the right way and not have to deal with expanding down the road.