
It also depends on how mature your build, configuration, and deploy scripts are. A start up with nothing will require more resources than a mature set-it-and-forget scenario. We have one release engineer for a ~25 engineering group. Our build kits are very mature so we are focusing now on expanding automated regression testing as part of the nightly build (i.e. selenium grid).
Even with a set-it-and-forget scenarion, there is always something to work on or improve.
Sean