I was looking at this video about Japanese Kanji on youtube, and found a discussion where people were talking about learning too many languages. Several claimed that they started to forget English because they had to invest so much time into Kanji. Some people were reporting the same problem from just trying to pick up too many. Others made claims like they started to get the languages and/or alphabets they knew mixed up, or they would start speaking English in a foreign accent, or they now seriously have an easier time speaking languages OTHER than English. And these are coming from people who only know 3 or 4.
I have heard stories of highly gifted people knowing over a dozen languages (and actually being fluent in 5 or more of them too). But how many can the average person seriously know? From what I've seen its rare for someone to know more than 2, but of course I'm American, its hard to find anyone here who isn't monolingual (aside for the Hispanic immigrants, but even a few of them are monolingual). I know my German teacher knew English, French, and German (her native language). She also taught a Spanish class, so I'm guessing she also knew at least some Spanish. She also claimed she knew Latin, but couldn't speak it fluently.
But really, how many languages can the average person know? I myself already know German up to an intermediate level (though I struggle to form my own sentences now, but I've been trying to find a way to correct that). I would like to learn as many as possible, but I don't know how many I can handle. I did have an issue when trying to study Japanese while I was taking a Spanish class (specifically my issue was both languages have an adposition called 'de', which means different things in the two languages), of course that was because I was trying to study two languages simultaneously.