Pfff, three and a half (lol, say three and a quarter) and I've barely studied any. I come from a bilingual region, plus english which is a must, plus some german. I'll start french whenever I can, and I'm still way behind where I'd like to be. It's relatively common if you live in Europe and move around just a little bit.
But the article doesn't refer to "learning" languages, but true bilingualism, which is harder to find/achieve. I heard a long time ago that it helps keeping your mind sharp and improves general abstraction, but I can't remember the details. Of course it doesn't hurt, but after all it's another boring test that finds its way to the press.