How does, "tu n'en as qu'une" mean you only have one?
It is from the popular French novel, ta deuxième vie commence quand tu comprends que tu n'en as qu'une. It's driving me crazy.
I'm guessing the main source of confusion here is the meaning of "en." This is a rather abstract descriptor that doesn't have any perfect equivalent in English. Using it before "avoir" will almost always give it a meaning relating to the quantity of something. Additionally, en cannot be used in this way unless a noun has been specified prior to its use. It more or less shows that the speaker is talking explicitly about quantity relating to the aforementioned noun ("vie" in the case of this sentence). Also, if "en" is used before "avoir", but there is no number afterwards, this means "some" or "any".
L'homme veut acheter des roses pour sa femme, mais il n'a pas assez d'argent pour lui en acheter. (This is just an example, but saying "pour lui en acheter" is redundant in this sentence)
The above sentence translates to "The man wants to by some roses for his wife, but he doesn't have enough money to buy her any."
I hope this helps :) If you still have questions, don't hesitate to ask.