Not a native speaker, but the sentence sounds to me like a comment to "Han gav mig ikke sit nummer, (må jeg bede om DIT nummer?)". Is the stress correct even in that case?
Could I say "may I ask your number". Maybe it's just that we abbreviate everything over here but "for" sounds a bit redundant
No, you can't omit "for," since that sounds like you are talking to the number. You ask somebody for something.
(There are a few special cases where this rule doesn't apply, though not too many spring to mind. For example, you can ask forgiveness -- though you can also ask for forgiveness. But you can't ask tolerance; there the "for" is required.)