A lot depends on how close your target languages are — both linguistically and geographically. If your native language is European, and you’re learning a couple of Germanic languages and a couple of Romance languages, and you live in central Europe, it’s not that big a deal. On the other hand, if you’re learning unrelated languages from the four corners of the world, you’d be much better off concentrating on one language at a time, especially if you’re not an experienced language learner. It’s most efficient to get a language up to a level where you can use it in real life, because then you can “maintain” it with your everyday activities, like watching movies and talking to your neighbors. It’s very difficult to “maintain” (much less learn) lots of languages that you cannot actually use in real life — either due to a low level, or simply a lack of access to people who speak the language. It’s OK to break the “rule” (I do), but it’s definitely not ideal.

Thanks Dari, good luck for you

I am learning three languages at the same time and l cant say that it is just waste of time.  I have hever thought that it might be possible for me, but the more languages  l learn the more it becomes easier to start new language and now l can hold conversation on all of these languages, more or less. Every day l study every single language. And the main problem is if l studied the only one l would have more time for certain language and for sure l would improve it faster...but the thing is l am not in a hurry...studying languages is my hobby, step by step l will succeed )by the way, l had similar opinion like you before. 

Thanks for detailed answer. You are absolutely right. But I said about person who writes - english, russian, chinese, ivrit etc.

Absolutely not. I might be a bit suspicious though if their level at all of these languages is zero, but even then, if someone just wants to learn a couple of travel phrases in many languages, I don't see a problem with that.