As I have mentioned before, those questions really have no answer. The appropriate ratio depends totally on context - the industry, the company, the software, the projects, the budget, the role of the testers, etc, etc.
But, for those who really crave a ratio, and don't care about context, the current issue (December 2006) of Better Software Magazine provides an answer.
Hundreds of their readers answered a survey about their employment situation.
In the results, they present several charts - one of which is the "Ratio of Testers to Developers".
While precise numbers are not given, their chart appears to show the following:
- about 5% report a 1:1 ratio
- about 45% report from a 1:2 to a 1:4 ratio
- just over 30% report from a 1:5 to a 1:10 ratio
- about 10% report a ratio of 1:10 or more
- just a few percent report 2:1, 3:1, 4:1 or 5:1 ratios
