How Many Languages Is It Possible to Learn? And this is one of many similar responses I got from Google, this one from The Linguist Blogger.
It appears that the only limit is the time it takes to learn each one. There's no limit to the number we can attain, and there are people who can speak 200 or more — the problem will be to retain them. The human brain has far greater capacity than any of us can ever exploit; it simply forms new neural pathways for every new routine we learn, whether a piece of music, computer code, dozens of PINs, or city map. Or a language. Wow. I'm impressed by my own brain (and by yours, too, and every human's). The amount of lore that a London taxi driver or a Mumbai dabbawalla can keep track of seems astounding — but they are just ordinary people like us, and if we went about it the way they do, we could learn all that too.
But how many languages can one usefully learn? If comparative linguistics is your thing, then maybe learning 200 will be important. For most of us, the answer is only as many as we need, and only as much as we need for our uses— whether as souvenir hawkers in a tourist center, nomads, foreign correspondents, diplomats, international bankers, or casual travelers, et alii. And, use it or lose it; if we cease using a language, we cease reinforcing or creating new neural pathways that let us find the word or phrase we need when we need it. But my experience confirms something in that blog post cited above: once learned, a disused language may not be totally lost.
For now, I'm hoping to be able to ask and understand directions, order food, etc., in Moscow and St. Petersburgh. And maybe even to have a conversation. Learning anything new is a thrill, and languages come more easily to me than, say, computer code or streetmaps or almost anything else. Language is a way into another person's way of thinking, and that's something we all need.