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.