It all comes down to your goals. If you want to learn fast, I recommend you only learn one language at a time. It's a matter of time, momentum and focus. Every minute you spend learning target language #2 is a minute spent NOT learning target language #1. This all adds up, and you end up making much less progress in each language than if you dedicate your time and limited focus to just one language at a time. I recommend getting your language up to a high B2/C1 level before transitioning to a different language. Language attrition is a real thing, but at a high B2/C1 level, you will likely have had enough time, repetition and experiences that your language has safely made its way into your long term memory. So if you transition to a new language, you won't lose much of your first target language.
Now, if you have lots of time and don't mind making slower progress, it is possible to learn multiple languages simultaneously. If this describes you, make sure your languages are from different language families. Don't try to learn Spanish and Portuguese simultaneously for example. This makes it so your brain is better able to compartmentalize the vocabulary, grammar and sounds of each language. Also, I recommend learning each language in larger chunks (Monday-Wednesday: French; Thursday-Saturday: Arabic) This helps give you more momentum and repetition in each given language before switching over to another target language. The biggest key is to have fun, do what holds your interest and keep yourself engaged