Cannot upload more than 20 images via a single submission of item form

The reason is that your server is posing a limit via PHP configuration variable:
max_file_uploads

1. Either edit you php.ini

max_file_uploads=100

2. Or edit you .htaccess

php_value  max_file_uploads  100

NOTE: if your server is using suhosin.upload.max_uploads
then add to .htaccess

php_value  suhosin.upload.max_uploads  100