"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.
If it was, say, your father's unit number in the marines, or your old house number, or something else, would you say "…ważnym numerem dla mnie" in that case?
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.
Hmm.. Why doesn't 'dziewięć dla mnie to ważna liczba' work? Is this completely wrong or does it just sound really bad?