Speak for yourself: I built on learning 2 foreign languages in highschool to end up speaking 7 languages (granted, only about 5 at a level of easilly maintaining a conversation).

The more languages you learn and the more you use them, the easier it is to add more languages to the pile.

Also, at least for European languages, because they generally are related, learning a few helps with learning others: for example, my speaking Dutch helped me learn German and there are even weird effect like me being able to pick up words in Norwegian because they're similar to the same words in the other two or when somebody gave us an example of Welsh in a trip to Wales I actually figured out he was counting to 10, both because some numbers were similar to the same numbers in other languages plus there is a specific rythm in counting to 10.

As I see it, the more languages you know, the more "hooks" you have to pick stuff up in other languages plus you're probably training your brain to be better at learning new ones.

That said, you have to actually try and practice them: for example, most of my French language was learned in highschool, so when I went to France or even Quebec in Canada I tried to as much as possible speak French, which helps with retaining and even expanding it so my French Language skills are much better now than when I originally learned it in a school environment.