You should just fill in the 16-digit Mastercard number and the other data requested such as the card's expiry date (probably indicated on the card as "valid thru") and of course your name as indicated on the card.
Perhaps (I don't know for sure as I don't know how the web-form in question looks like, but it is not unusual) you must additionally also fill in (in a different box !!) a three digit number which is on the back of your card (the last three digits printed in the space for your signature).