[bsb]

Polyglot should not be confused with generalists. There are many generalist programmers who drift through their careers, never becoming an expert in a field so they're easily swapped in and out, easily outsourced, and easily forgotten. Some of the specialists out there, can make some serious salary (e.g. 200-250k/year for those that can build high speed trading platforms, or 150k+ for the architects of the biggest enterprise software).

That being said, limiting yourself to one language will be equally as hurtful, as you are wearing blinders. Even if you don't know all the languages, learning why they did things (DRY, functional programming, etc) can make you a better programmer in Java or whatever language you are in. As you supplement that, you'll find overall skill increases across the board.

My personal opinion is about 1 a year will do just fine, give you enough room to learn more than a book/sample project worth, see where the language goes in a market, and give you enough time to use it for something meaingful.