What it means to have a sail and sign card is that they want you to give them your credit card number for the charges you may have to your stateroom. Everyone in your room can be on the same credit card if you are paying for everyone. If not, everyone in the room can use separate sail and sign cards. You don't need to make a deposit to your credit card unless you purchase items onboard like, bar charges, shore excursions, alternative restaurants, or shopping. You can always pad the account with cash if you don't want charges on your credit card.
Ive been on 14 cruises and I usually spend about 250.00 in one week onboard. I am not a big drinker or gambler but I do go on shore excursions.