Ethnologue states that there are 7105 languages in the world[0]. Therefore, it seems reasonable to assume that you can have x second languages, where x is any number between 0 and 7104.

[0] https://www.ethnologue.com/statistics

No, you can have only one second language, and then possibly third, fourth, and so on.

Not true in idiomatic English. Any language other than your 'first' can be called a 'second language'. As a common example of this usage, courses to learn English for non-native speakers are called "English as a Second Language (ESL)" - no matter how many other languages the students already know.

Then I guess only native English speakers can have many "second" language. In many other countries there are 1st foreign language and 2nd foreign language classes in the school. And those books are called "English as a foreign language" (Englisch als Fremdsprache).

[A second language or L2 is any language learned after one's first language.](http://en.wikipedia.org/wiki/Second_language)

Otherwise you introduce ambiguity, if I learned 50 words in Spanish and then mastered French, does that mean French is my third language?