There's no way the amateurs are hitting 17-18 greens, IMO. Take for example #10, #11, and #12. Without looking, I'm positive that there were several examples of both pros in a pairing being in the fairway, and both missing the green. Or both pros missing the green on 12 despite having the ball teed up.
Only 24 players hit at least 60% of the greens in the Masters this year, with the leader hitting 71%. Scheffler hit 68%. Collin Morikawa, the best ball striker alive currently, had a round where he hit just 9/18 greens.
The leaders for fairways were just over 80%. But some people, really great golfers like Justin Thomas and Sergio Garcia, only hit 60%.
These amateurs aren't the ball strikers that these major winning, top 25 in the world guys are. They're neither as long nor as accurate.
Let's assume that on their own, each of these amateur guys could manage 60% fairways out there. That means they're still, even when scrambling, going to probably miss 2-3 fairways. Let's say they miss 3. Now they are firing from the fairway on 11 holes, and from the tee box on 4 more, so they'll have 15 clean opportunities at a GIR. Let's pretend that on their own, these guys would each hit 75% of the GIRs from the fairway or tee box, which is already being generous. That means odds are, they're still missing at least a couple, plus they'll likely miss one or two from off the fairway.
In short, 14-16 GIRs would be their max, IMO. Hitting 16, even when scrambling, would be a huge accomplishment. I think they'd hit 14ish on a normal day, assuming they were playing well. Especially when you consider the uneven lies.
Some of those greens are really hard to hit, even for the best in the world.
None of that is accounting for the pressure of playing on that stage, either.