I’d like to know what “all key owners” refers to

This would be anyone who has gotten download access either from a purchase or from receiving a download key. The number depends on the number of keys you generated and handed out.

I read in the guide that if the game is sold for free (like ours in this black friday) it does not generate keys

This is true by default, but, if you enable “claiming” on the sale then keys will be generated. The people who downloaded it for free during the sale would be permanent owners. I checked the sale you did for black friday and you had claiming enabled, so all those people got keys.

I would also like to know if we can also contact followers via the email system

We don’t support direct emailing followers, you have to use the devlogs system. This is because of email consent rules. Your followers haven’t consented to being on a mailing list for you, but generally they’ll opt into itch.io email notifications. This means that posting a new devlog will generally send them the update.