How many languages can fit into a brain?

I asked my grandmother, who speaks German and English, which language she thought in. She said both. She said that usually nowadays she thinks in English, but when she's thinking about certain subjects or certain people, it switches to German automatically. I found the whole idea very interesting.

I also think there's no real limit to how many you can learn - but I think that most people (with people like that 80 year old woman being an exception) would have a difficult time RETAINING 13 languages, because they surely wouldn't use all of them on a daily basis. I think that languages are much easier to pick up and retain if you are exposed to them and use them on a daily basis. If I sat home and learned 4 languages from a book without ever visiting a country where they are often used, I would probably forget much of the vocabulary right away. I think the ceiling involved in learning is not necessarily how much you can learn, but how much you can retain.

Jam it back in, in the dark.