What is the ratio of developers and testers?

This ratio is not a fixed one, but depends on what phase of the software development life cycle the project is in.
When a product is first conceived, organized, and developed, this ratio tends to be 10:1, 5:1, or 3:1, i.e. heavily in favor of developers.
In sharp contrast, when the product is near the end of the software development life cycle, this ratio tends to be 1:1, or even 1:2, in favor of testers.