I think it would be fairly easy to be fully fluent in four or five languages if one lived in the right settings - Switzerland, for example, or Finland. The problem is that maintaining fluency requires constant practice, and so most people won't maintain that level in a large number of languages unless they're, say, UN interpreters, and even they usually specialize in two or three languages only. My Chinese back in law school was good enough that I took a class on Chinese law in Chinese (no English in texts or conversation), but right now, after about ten years of non-use, my Chinese has diminished considerably. However, when I did go to China on business back in 2007 most of it came back in a remarkably short period of time. Of course, on returning to the US most of it sank beneath the waves of the Lethe again (sorry, it sounds pretentious in English but I'm thinking of a common Russian phrase - кануло в Лету).

As it is, Russian alone has been tremendously useful for me in business.