Just a little confusing...

[Silent carillon]

1."Gently scratch the grey area below to reveal 12 digits card number." I got this caution in a phone card and I think there's something wrong with this. In my opinion, it should be "12 digit card number" to form a compound noun or "12 digits of the card number". Will you please give me a piece of advice?
And these 2 following are quoted from my exercise book..
2. "You'd better make your mind up quickly." Then how about "You'd better make up your mind quickly." or they're both correct?
3. "I don't know whether or not she's telling the truth.". Then how about "I don't know whether she's telling the truth or not." or again, they're both correct?
Looking forward to your reply. Thanks in advance.

[Tdol]

1- I'd say 12-digit
2 They're both possible
3 They're both OK, though I prefer whether.....or not.