So how long was Missy collecting human minds to be uploaded into Cyberman bodies? As long as humanity has had the concept of an afterlife. So maybe Missy is actually the reason people believe in an afterlife.
This is why I hated the Great Intelligence killing the Doctor at "every point in his timestream." It's the limitations. We know it did something to the Doctor during The Invasion of time, The Five Doctors, and possibly Dragonfire, because those were the clips shown, but the idea of being at every moment is difficult to visualise anyway. And this is the similar problem here. Among others...
1. Honestly, because of decay and other factors, I do not think there are enough dead bodies available now for as many minds as would equal the number of people who have ever died on Earth. When Missy says "every man, woman, and child who ever lived" I don't believe that's exactly true. This is where you have to balance between literal talk and poetic talk. Because of CGI limitations, or whatever, there does not look like enough Cybermen in that graveyard for as many graves as there were. I think there was mention of some graves reacting to the rain quicker than others, but I don't remember. I think Missy has been selecting minds that go to the Nethersphere. But then, if you can upgrade bodies into Cybermen do they need minds if you control them anyway?
2. Has everyone who ever died been uploaded into the Nethersphere? Because if that's just an automatic thing, that means that Dr. Chang didn't die and later became a Cyberman. And if it was still active between raining and Danny going back there, then that means Osgood is in the Nethersphere.
3. The way Missy describes uploading dying minds and upgrading bodies made me think that had to do with how the Master became the Mistress. Perhaps the fact he absorbed the life energy of women in The End of Time played a part.
4. I don't like the cyberised Brigadier simply because of what it is. But then it's only implied, and then later stories could always retcon anything they want. But does this mean other companions and friends who have died and would be buried on Earth became Cybermen? It goes back to exactly how many graves were affected.
5. I'm not trying to harp on this, but it's like showing vs. telling. Danny died in the sky, but then his mind (and body?) were returned to the Nethersphere along with the boy (assuming he became a Cyberman) and had the chance to come back. So, was Seb once alive?
6. Two people constitutes going up and down a timeline... Did they become Cybermen?