Here's exactly how the mining mechanics work in sc2.
1) only one worker can mine from a mineral patch or gas geyser at a time
2) workers phase through each other while mining, so they don't lose time pathfinding around fellow workers.
3) every base has 4 "close" mineral patches and 4 "far" mineral patches. The close mineral patches are mined at full efficiency by only 2 workers; as soon as one finishes mining, the other has already returned and starts mining immediately. For the far patches, when one worker finishes mining, the second worker is only halfway to the minerals. Sticking a third worker on a far patch can cover that little mining gap, but the benefit is small (like ~30%).
4) therefore for efficient mining you want 16 workers on minerals. For "perfect" mining, you want 20 workers on minerals with the extra 4 on far patches only.
5) usually a triple-stacked close patch will automatically lose the excess worker to a far patch, but sometimes you need to "worker stack" manually to achieve this. You can see MaNa worker stacking manually at the beginning of his showmatch when he doesn't have anything else he could be doing with his excess apm. 24 workers on minerals guarantees maximum income because 4 workers will never fit on a patch; they'll start automatically reassigning themselves to different patches until they find a less crowded one.
6) more than 24 workers on minerals provides exactly 0 extra income. Still good to have extra workers though so you're not losing mining time when you build buildings, get harassed, etc.
7) every base has 2 vespene geysers, and 95% of the time these are in a position where 3 workers will saturate them perfectly. There are a couple of maps (or at least used to be... might be fixed now) where you can eke out the tiniest bit of extra gas by having 4 workers on a geyser.
8) every serious sc2 player already knows all of these mechanics
I personally think Artosis was overstating the innovation of DeepMind's worker over-saturation. I agree with something Rotty said earlier in the series; sometimes you make a suboptimal move and it works, but that doesn't mean it was the "correct" play. It just means that you papered over a flaw with stronger execution somewhere else.
I'm still very impressed with these games, just not with regards to the AI's build orders.