It all depends on your situation, how complex the software setup is, how the development is being done, whether the software is operated in-house or by an external party or delived to end-users, etc. Do the testers work closely together with development? Are there separate test systems that needs to be taken care of etc. Is there custom hardware involved?
But in my experience one tester to ten developers seems a bit low.
Something that have worked well for us is to have the testers and developers at the same level, both reporting to the project manager. Each iteration the software is stabilised and programmer tested before an internal release to the testers. They then deploy the software in their own environment.
To get this to work, it's important to practice the release process during the early iterations (short iterations with almost no features) to get all the not-so-obvious problems out of the way. It also helps the testers to get familiar with the basics of the software, and to identify where some things may need more operations documentation.