I've worked in numerous environment, and the ratios have been all over the place. One place I worked we had no DBA, but easily 20+ developers. Although, there was probably only 7 or so of us who did any SQL work. We likely didn't have a DBA because we were developing a product which used SQL Server. So, there wasn't anything for a DBA to maintain, as the databases where all on develop laptops or workstations. There was one place that was on the order of 1:10. Numerous other smaller shops (< 6 devs) we had no DBA. My present employer has 1 DBA. There are 22 "real" developers and I'm not sure how many implementation specialists who also do some simple SQL work. So, the ratio here is 1:22+.
MKE Data Guy