Apple ships close to 200 million iPhones a year all with their custom ARM CPUs, fabricated by TSMC. They ship about a tenth as many Macs. I'm pretty sure they can handle that growth, especially as one of TSMC's biggest customers.
And the move to 5nm for the A14 (sampling since 2019) will yield more chips per wafer. Either way, Apple is very forward looking on supply chain and has been planning this for a decade. Remember when they owned the majority of the nand market due to long term supplier contracts?
Interesting thought. Will the functional yield really be much higher? Because there could be more wasted chips?