What is the proper number of languages someone should learn and is learning many languages at a time counterproductive?
for me learning 3 languages is what I can for now. first is my native tongue, 2nd is english and my 3rd and fourth is Japanese and Korean. I think learning a language should have a really deep reason on why you should be studying it. I don’t know for others but learning a language is more than memorizing vocabulary. We can memorize lots of words but speaking and listening is another thing. For me learning one language at a time is enough. Because is you really want to learn to it, you should immerse on it.
For most people, any number, but best to do one at a time.
That said, there are circumstances where you could learn several at once. But you would need to know what you are doing, IMO.
If you want to learn more about how to approach several languages at once, you should check out Frederick Bodmer’s famous book: