We currently have 4 developers and I'm the 1 DBA, which seems like a good ratio. The last company I was at had 8 DBA's and 30-40 developers.

Of course, there is a pretty broad spectrum of what duties are assigned to a DBA. I tend to notice that when you get past a 4:1 ratio, you see the developers writing their own DDL and the DBA tends to be more of a systems administrator. I find that I'm mostly writing SSIS packages and troubleshooting performance issues these days, and I dole out advise on data modeling and the finer points of writing SQL, but very little actual code writing.