As with everything else in life, this is a compromise. It's a compromise of programming knowledge vs. the time to learn it and the time to keep learning. 10 programming languages, like others have mentioned seems exorbitant and you'll likely be spreading yourself thin. Knowing a functional language, an OOP language or two should be sufficient as your core languages. This isn't to say you shouldn't dabble in new languages and frameworks as they come out, but unless you're willing to cross bridges, the amount of time you'll spend learning the language, getting accustomed to new tools etc. will cost you more in time than is worth it to keep up to date with your core languages (especially if they're as fast-paced as Ruby).