It couldn't because that would make it "the number by my account" But you could say; "Her er mit kontonummer"(Here's my account number(quicker and more popular)) or "Her er nummeret til min konto"(here is the number for my account)
Oooh okay, I see... So if I want to say "my father's bed", should it be "min fars seng" or "sengen på min far"?
Min fars seng is the way to go here. The other version sounds like you father is wearing the bed as a hat.
I think the difference here is because the number isn't an actual belonging of the account, but merely attached to it. Like "it says this number on the top".