Anywhere between 1:7 and 1:40. The later was a pure admin type DBA job, the ones with the lower ratio tended to also involve a lot more development and architecture work.

My previous company was most likely the worst. 82 production databases, with dev and test environments for each, 22 developers in house AND supporting an off shore team that could be anything between 5 and 40 people depending on what project was going on. With me as the only DBA (most of the time, for a while had a backup DBA). I was the architect and production DBA, luckily the developers had pretty good T-SQL skills. I quit because of the hours needed to support both on and off shore.