This year, due to the COVID-19 pandemic, we did see Apple's annual iPhone launch delayed. The iPhone 12 was pushed out of the usual September launch window and into October. In 2021 though, we should see Apple's schedule return to normal.
According to Apple analyst, Ming-Chi Kuo (via 9to5Mac), Apple will begin mass production for the iPhone 13 earlier than we saw with the iPhone 12, bringing things back in line with previous generations of iPhone, most of which launched in September.
This essentially means that we shouldn't expect manufacturing delays to hit the iPhone line-up in 2021. All iPhone 13 models should launch in September next year.
Kuo also anticipates that Apple will be sticking with TSMC for SoC production. TSMC has already outlined its shipping forecast for Q1 and Q2 2021, which will see shipments of the A14 decline. This is a seasonal decline, not tied to direct iPhone 12 sales. As A14 chip production declines, Kuo expects A15 SoC production to ramp up.
KitGuru Says: There'll be plenty more leaks surrounding the iPhone 13 in the months to come, so this is just the beginning. In the meantime though, the iPhone 12 seems to be doing well for Apple, with strong sales for the Pro model in particular, which is apparently selling better than expected.