limitation on "add more" button: Drupal 7
Sounds like you're after the Multivalue Settings module.


  So you can create fields where, for instance, users have to input at
  least 5 values but can input up to 10 values. Or you can create fields
  that allow 100 values, but only display 20 fields to start with.


HOWEVER (and this is a big however - if I could change the font size I would)

This module should probably only be used as a starting point, and should not be used in production. I'm sure you can get the code you need from it. 


  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. I'm
  working on fixing things, but don't have much time. Help is welcome.