How many DBA's should a company have [on hold]

How many DBA's should a company have? Does exist any rule of thumb?

I am working on a financial services firm with 500+ employees and 200k+ clients. At the present time, data warehouse is on another area and so does server and infrastructure. How many DBA's should be working considering we have many critical DB engines with a few db's each, and many other non critical but important db's for daily operation plus all testing/development db's.

Regards