They've already got multiple 20nm fabs
From a story about TSMC 20nm production: "At present TSMC is producing chips using 20nm fabrication process at select modules of fabs 12 and 14. Starting from May, the company will initiate 20nm production at the fab 15 modules 3 and 4."
That's more 20nm fabs in May than Samsung has 28nm fabs today, so Apple would already be more resilient to fab problems with TSMC than if they stayed with Samsung. The one advantage of Samsung is that they share process technology with IBM and Global Foundries so there would be some overflow if Apple needed more production than planned, but TSMC is larger than all three put together so that's not a problem either.
Of course, this all assumes that the rumors are true. Apple has been rumored to switch to TSMC for a couple years now, no one will know for certain until the weekend after the iPhone 6 is released and IHS does their usual teardown analysis to see who made what parts and estimate the BOM costing.