Anywhere from zero to 1000 (or 1500 on the new ships) depending on your intention.
There are 4 tipped positions...so if your intent is something a little extra with those, the answer would be 4. If you have kids in the programming and want to do something for them, a group gift is more effective as your child may interact with 20 or more CMs during the cruise. If you are one of the people who hand out gifts to random CMs on deck or whatever, the number is totally up to you. I'm not saying this is a bad thing--these are the yellow jump suit guys who are rarely rewarded.
Some people leave stuff at Guest Services for a particular department--engine room, theater tech crew, etc....another way to do something for groups that are rarely recognized.
Bottom line--nothing is expected beyond cash tips for the 4 tipped positions. Anything beyond that is totally up to you. People in non-tipped positions are not allowed to keep any cash that is given to them. They MUST turn it over to their manager or to Guest Services or risk job loss. They can keep small gifts like a candy bar or whatever.