Is it advisable for one to specialize in one programming language and be really good at it or one must know quite a number

Answer 1

It is good to at least be an expert in one or more languages. You wouldn't want to end up being a Jack of all trades.

Answer 2

Both: Be really good at one language so that you can contribute efficiently and help others. Then also build up some experience in related complementary techniques so that you understand what is going on around your area of expertise.