Image field: Force at least/exactly two images per field
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You can do it by using Field validation Module.

The following validators are listed:


  
  Regular expression
  Numeric values (optionally specify min and / or max value)
  length (optionally specify min and / or max length)
  number of words (optionally specify min and / or max words )
  Plain text (disallow tags)
  Must be empty (Anti-Spam: Hide with CSS) 
  Words blacklist
  number of selections (optionally specify min and / or max selections
  )
  Unique
  Match against a field
  Match against a property
  Specific value(s)
  Require at least one of several fields
  Equal values on multiple fields
  Unique values on multiple fields
  PHP Code (powerfull but dangerous)
  URL (support internal path and external url validation)
  Email
  Pattern (Regular expression lite)
  Date range2
  


Hope this will help you