If you are running a WordPress website and are having issues whenever you try to upload more than 20 files, then update the max_file_uploads php directive in your php.ini file.
This limit may get exceeded if, for example, you are adding items to your media library and select too many files to upload in one go. It may also occur if you have a website form that allows visitors to upload files.
You may need to ask your website hosting company to do this for you.
Bear in mind that the limit is there to protect your site, and your hosting environment, so think carefully before you increase the file uploads limit, and only do it if there is a genuine business need AND you can trust those who are uploading to your site.