How many accounts in one server?

We do not have specific number of accounts in one server, because we aren't usual hosting company which goal is to put as many customers in one server as possible. Our first and furthermost goal is server stability and performance, so we're keeping average CPU IDLE time around 60%-70%. This gives us enough headroom in case some website sees a burst of traffic or some heavy tasks are running, eg. backups or large amount of files are moved.