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.