Not department, project. If you can get to 10 across a whole company that would be good, but project unreasonable in the largest companies. Even with only 50 people in a startup staying under 10 will need some effort as there is always some cool new toy. (don't let them replace languages with frameworks either)
This is important. Sometimes you have multiple departments working on a single project (a large embedded system). Sometimes a department will work on multiple projects - the department needs to be careful that people don't need to know too many languages (which is hard - if often make sense for a department to develop for all apps, so you have to know the language of iPhone, android, Window)
People should stay mostly in their lanes (department), but they should have the ability to cross to others when that is needed instead of just throwing APIs over a wall and accusing the other side of using it wrong.