I think it’s 16.
If we get the same 15 runed per T11 we did in S2 then it’s absolutely faster to just grind fungal Folly or similar than do keys, especially if I’m having to sign up for them.
Maybe a really good premade can go faster but even then you’re now relying on 4 other people playing like machines. Not to mention fungal Folly is basically gear agnostic thanks to the spores, meanwhile you actually need ilvl and score to do m+ at levels that award competitive amounts of gilded.
Last season when crests unlocked I just did fungal Folly for about 2 hours and was completely done upgrading my gear.