Reported tester: developer ratios range from 10:1 to 1:10. There's no simple answer. It depends on so many things, Amount of reused code, number and type of interfaces, platform, quality goals, etc.

It also can depend on the development model. The more specs, the less testers. The roles can play a big part also. Does QA own beta? Do you include process auditors or planning activities?

These figures can all vary very widely depending on how you define 'tester' and 'developer'. In some organizations, a 'tester' is anyone who happens to be testing software at the time -- such as their own. In other organizations, a 'tester' is only a member of an independent test group.

It is better to ask about the test labor content than it is to ask about the tester/developer ratio. The test labor content, across most applications is generally accepted as 50%, when people do honest accounting. For life-critical software, this can go up to 80%.