Can't see such a launch from Apple happening. If TSMC pushes Apple to such an unavoidable choice I expect Apple to think twice about future cooperation with TSMC.
Whatever happens we should expect TSMC's close customers (as at least Apple, AMD and MediaTek are) to be privy to any potential internal issues as they crop up and work together on possible workarounds and alternate solutions accordingly.
Yep 0% chance that Apple launches with a "more limited supply" due to their own SoC, when they work so closely with TSMC they help define each new process. They would know far in advance if there are any problems, and would be able to retarget their process. Heck, its possible they might have a dual track design process for EVERY new A series SoC, with a fallback option available just in case. Considering how many A series SoCs Apple ships, doing such a backup design would cost them like 10 or 20 cents per iPhone. At least it would make sense when jumping to a new process, i.e. for the A14 designing a fallback option on an N7 variant and abandoning it only when they knew N5 was fine.
Anyway, it has been public for a long time that N3 would not be ready in time for Apple so is the claim now that N4 will somehow have a "limited supply"? Or that despite working alongside TSMC and knowing the N3 schedule long before they become public, that they've stubbornly designed A16 on N3 and only now are saying "oh crap, it isn't going to be ready in time"?
Seems to be a lot of people pushing this idea that TSMC is suddenly turned into Intel and is unable to get a process working, while taking Intel's roadmap claims as gospel. I guess there are some here who are heavily invested in INTC or perhaps are holding a lot of underwater options, and really need that stock to make a comeback, which requires a big TSMC stumble to cut the legs out from under AMD.