Hello Ash4343,
Thanks for asking about 20 unread emails, but when you check the app, nothing is there. You mentioned this is iCloud. Check by using a computer and internet browser, and go to iCloud.com.
Check if your emails are showing there. This might happen if you are out of iCloud storage: Manage your iCloud storage - Apple Support
"If you run out of iCloud storage, your device won’t back up to iCloud, new photos and videos won't upload to iCloud Photos, and iCloud Drive, other iCloud apps, and messages won't stay up to date across your devices. And you also won't be able to send or receive emails with your iCloud email address."
Once you clear them from iCloud online, see if it syncs up properly on you other devices. You may need to make more room available in iCloud depending on how much space you have, and how much storage is taken up.
Thanks.