7 gens done in one match
This is the challenge:
I got 7 gens done in one match, according to this. I find interesting the way this challenge works. It counts the 2 extra gens that get automatically completed on the map once you complete the requested 5.

Comments
Its fixing the equivalent of gens not a single gen on itself.
You can fix one gen to 99%, then the Killer comes, kicks it, regresses to 0%, you fix it to 99% again and you will have progressed the challenge by almost 2 gens because you fixed the equivalent of almost 2 gens.
EDIT: My bad, I read the challenge the wrong way, what I wrote doesnt apply to it, yes its weird it counts the gens powered when the gates get powered.
Honestly, this seems like a bug. Nobody is really repairing those gens, they just finish whenever the fifth is completed.
I mean, as long as it gets fixed.... :P
My guess is that if other survivors are in the repair action on the other gens when the final gen is completed and those gens get auto-completed, it counts those gens as being finished by those survivors.
That actually sounds pretty likely tbh. I wonder if it's been that way forever or this is a recent thing? With BHVR and their spaghetti code either is completely believable.