Welcome to Keymailer!
Every Dev is checking for themselves, so I can't 100% tell you how everyone is checking on who to give keys to.
Out of our experience:
As soon as you request a key (and are accredited) your account shows up in their requested list.
Here they have a overview of your profile and your subs on YT, Twitch, Mixer and Twitter. As well as a ratio that shows up if you cover keys you receive or not (Its a rating going from -3 to +3).
They can go onto your profile and see all of the videos you have produced and which games you have played as well as how many hours of gameplay you have on your steam account to which genre.
On all of those numbers they then decide on who to give a key to and who not.
As soon as they gave you a key and produce content it shows up on their side how much content you produced and on which platform and they have a direct link to the video/stream/tweet to check it out.
There is no generalilzation on how much content you need to produce for a key, whatever is okay for you. You don't need to produce 2,5,10 videos for a game that you do not enjoy playing, that wouldn't be fun to you either. If you want to, you are free to do so, but thats all up on you.
Also you do not need to cover every single game you are receiving, just because you do not cover every single one, doesn't mean you go down to -3 immediately.