The testing environment and infrastructure should not be under-estimated when deciding the developers to testers ratio. Software development companies that are more into manual testing, need more testers as compared to those who have automation testing tools.
As we know, no manual or low manual effort is involved in automation testing and we get great results in return. Hence, in this case, a lesser number of testers will also work.
But if the company or project is manual testing dependent, the number of testers should be more. Also, during manual testing, we need multiple mind-set in order to cover all possible conditions.