Minimum number of languages that a developer should master?
In any case, whenever I hear someone blasting Java, I know that they likely don't have real experience developing large scale Java server side applications and don't understand the benefits of the server JVM and a huge collection of good tools and frameworks.
Whenever I hear someone knocking scripting languages like Ruby, Python, or Perl I always think that what a shame it is that they don't save themselves a lot of development time using an agile scripting language - when appropriate.
Then there are times when it is "just right" that development be slow and painful, and C++ is used for runtime speed and memory efficiency :-)
Also if you need a carpenter to do some maintenance on your home: don't hire a carpenter who only uses a saw.