How many languages can a normal person (with a life) hope to learn well?
Cardinal Mezzofanti whom Byron called “a monster of languages, the Briareus of parts of speech, a walking polyglot, and more” and Emil Krebs who learned Armenian in 9 weeks are rare exceptions. Krebs reportedly spoke 60 languages. He was a professional interpreter and linguist. Krebs is significant because he donated his body to science and his brain was analyzed. The scientists discovered that the brain’s Broca’s region, which is associated with language, was organized differently than that of other people. Did he work on languages for so long that he altered his brain or was he genetically predisposed? An interesting question but of little practical use for most people.
What are the limits for a normal person, taking into account things like an 8-hour work day that is not necessarily related to foreign languages or generally speaking a lifestyle that is far from the tranquil surroundings of a monastery? We want to target a number that will be manageable as we might conquer 10, but eventually only defend 5. So, how many? Scientific studies concentrate on how we learn languages – not on how many we can possibly learn.
There’s no research that I’m aware of except for a book by a certain Dr. Dmitri L. Spivak: Kak stat' poliglotom - How One Becomes a Polyglot (1989). The book is recommended by Erik V. Gunnemark, the co-author of The Art and Science of Learning Languages. Unfortunately there are only a three or four translations of this book into other languages and none of them are in English. The book features among other things several interviews with polyglots from the former USSR. Most of the interviewees agreed that they were not able to speak and read more than about seven foreign languages. He called this “The Law of Seven.” The "law" or more likely "rule" states that most people can learn between five and nine languages.
These guys likely had above-average aptitude and more importantly a backbone and a willingness to sacrifice a lot of time on language learning. They attempted to learn more but were able to learn “completely” about seven languages. A lot of the languages some of them learned or attempted to learn were related but the number of languages they were able to learn remained the same.
In his book Why You Need a Foreign Language & How to Learn One Edward Trimnell indirectly mentions Spivak's Law calling it the "Rule of Seven" and says that "there is a commonly accepted notion amongst polyglots that seven languages is the maximum that a person can learn to a high level of fluency." He also mentions that the Rule of Seven proscribes that seven foreign languages is the "practical upper limit" for most people. Spivak's uppermost limit is nine languages. Trimnell then suggests that the upper number is actually far less for those who are not professionally occupied with languages - something like three languages. He reached this conclusion based on online profiles of the members of American Translators Association and their working languages. While he found that it was common to find translators with three working foreign languages, he was unable to find translators who worked with more than five. While I do take objection to Mr Trimnell's idea about what constitutes foreign language proficiency (translating and interpreting requires a very special set of skills) he does offer some very useful advice. Among other things he urges the part-time linguists to make up a list of languages that interest them and narrow it down to three.
Kato Lomb, from the foreword to the first edition of her book "How I Learn Languages":
“...it is not possible [to know 16 languages]—at least not at the same level of ability.”
“I only have one mother tongue: Hungarian. Russian, English, French, and German live inside me simultaneously with Hungarian. I can switch between any of these languages with great ease, from one word to the next.
“Translating texts in Italian, Spanish, Japanese, Chinese, and Polish generally requires me to spend about half a day brushing up on my language skills and perusing the material to be translated.
“The other six languages [Bulgarian, Danish, Latin, Romanian, Czech, Ukrainian] I know only through translating literature and technical material.”
Emil Krebs in 1913:
Englisch, Französisch, Italienisch, Spanisch, Russisch, Ungarisch, Chinesisch beherrsche ich außerdem derartig, dass ich aus dem Deutschen in diese Sprachen korrekt Übersetzungen anzufertigen im Stande bin. Auch im Finnischen habe ich soviel Übung, dass ich mir zutrauen kann, aus dem Deutschen ins Finnische Übersetzungen anzufertigen, die den Sinn des Deutschen verständlich wiedergeben.
In short Krebs was extremely proficient in nine languages - including his native German and Finnish. He thought it necessary to single out Finnish and mention that he believed he could "also" translate from German into this language. Translating into a foreign language requires a very high set of skills.
Kato had extreme proficiency in 5 languages. She was also very proficient in the other five that she mentions needed half a day of brushing up.