Can’t Upload More Than Some Specific Number of Images at Once
If you cannot upload more than some specific number of images at once for product options or option combinations, then it may be because of a PHP setting that limits the number of files that can be uploaded at once. That setting is called max_file_uploads.
The default value of max_file_uploads is 20. It means that the administrator of the store can upload no more than twenty images at the same time for product options, and no more than ten images at the same time for option combinations (one additional thumbnail and one larger image).
To resolve the issue, increase the value of the max_file_uploads setting on your server.