Make drupal cck field behave as "an add more button"?
This functionality is not available out of the box. But it's something people have been asking for (repeatedly) since at least D6 CCK 2:


Enable 'add more' button for non-unlimited fields.
Provide "Add more values" button for fields (and multigroups) with fixed number of multiple values
'Number of repeats' uses 'Add more' button
"Number of Values to Display" option for CCK fields


User goron has developed a contrib module for Drupal 7 called Multivalue Settings.

But be aware - from the module description page:


  Warning: this module is still very buggy. Actually, it only works in
  very few use cases. I suggest to only attempt using it with core
  fields (not fields from contrib modules). Even this may not work.


And it hasn't been updated in over a year.

Screenshot of Multivalue Settings functionality: