It doesn’t matter how many days before the due you pay
As long as you pay before the due date – it’s good enough. It makes no difference if it’s 1 or 20 days before.
Just make sure you pay after you get the statement. If you pay your balance before the closing date, your statements will show $0 balance. Since the credit card companies send their data to the credit bureaus at the closing date each month, if your balance is $0, then it will have the same effect as not using the card at all, and that WON’T build credit for you.
So wait for the statement, and pay in full any time before the due date.
As to the number of credit cards, having another credit card may help a bit. Having a different type of credit (e.g. car loan, personal loan or mortgage) would help your credit much more.
The BEST way to jump start your wife’s credit is to put her as an authorized user on your credit card. It’s called Credit Piggybacking, it’s perfectly legal and Fico acknowledges it. Your credit card will be reported to both of your credit reports, and will build credit for both of you.
Using ITIN to get credit is against the law, and can get you into trouble. If she has no SSN, she needs to use her state’s ID instead.
Hopes this helps