How many minimum languages should be known by a good programmer.

Answer 1

Master in one language + some latest technology will help to achieve your goal..

Answer 2

It depends on the work you do.

Occasionally there are specialists who mostly work in one language, and in other cases people switch between all sorts of languages, frameworks etc. as the need arises.

Don't stick a number on that, it doesn't lead you anywhere.

Instead, ask yourself what exactly you want to code, then learn exactly what you need for that.