"Nine is an important number to me."
Translation:Dziewięć to ważna liczba dla mnie.
OK, that probably should work, so added it.. But generally, in maths, "number" = "liczba". "Numer" is mostly an ordinal number, like number of the house, of a locker, of a room, reservation, order etc. Also a telephone number.
So your version sounds so-so, it's rather on the verge of acceptability. But okay.
Frankly I still would, personally, probably use "liczbą", because even if "7" was my locker's number, house's number and my number on the list of the class, it's still "moją szczęśliwą liczbą". (my lucky number).
The only situation, when at least right now I think I'd use "numerem" is a telephone number, like for example my old number from the telephone I lost.
OK. I need to rant here for a few seconds. One my pet peeves with this course is being expected to know, without any contex, what is the most important idea in a sentence and to put it in the correct position in the Polish sentence ( i.e., do not use the English word order as a guide.). The other is expecting to know what "sounds bad" in Polish without much explanation of what "sounds bad" means in any given situation. I tried what Philipp88107 proposed because in other sentences "dla mnie" seems to work fine in Polish in the interior of the sentence, even when the English version would have it at the end. In this sentence, however, it now sounds as bad as the English version. In fact, it is so bad that it cannot be correct and is not an acceptable Polish sentence. After working on this course for about two years, I have not developed an "ear" for what sounds good/bad in Polish. It is frustrating to say the least.
I strongly feel that a few 'Emphasis' or 'Stresses' lessons could be added to the skill tree, not only for Polish but also for other languages (Hungarian is another prime candidate). Have an English sentence with the emphasised word in bold, and give three translations with differing sentence structure. These lessons could also introduce the stressed and unstressed forms of pronouns (e.g. ciebie/cię) in a more systematic way. Different skills could deal with emphasis when negation or contrast is involved.
@Jellei, would love to hear your thoughts on this :)