I can upload 20 images only

There is a value in the php.ini file which is called "max_file_uploads". It limits the number of file upload fields that can be handled on a single page. Please increase this value to something you need, and restart your web-server (Apache).  Generally, you can add the line in configurations:

php_value max_file_uploads 100

to the .htaccess file in the magento root folder.

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