Is there some source or reason you think there will not be a AS Mac specific processor or is this just speculation?
Speculation.
TSMC Fab 18 started full-scale 5nm production this summer and Apple has booked all of Fab 18's production for the remainder of 2020 (and an indeterminate period beyond 2020). Production at full-scale is said to have started around 50,000 wafers a month and will be around 90,000 wafers a month by the end of this year. Projected net yields for the 5nm process are around 500 SoCs per 300mm wafer so TSMC is producing around 25 million A14 SoCs a month since late summer and by the end of the year should be around 45 million a month. However, Huawei bought up a significant amount of early 5nm production so that would probably have put some limitations on how many SoCs Apple could secure before they signed the agreement to take all remaining 2020 production capacity.
A new iPhone usually ships around 75 million units in its first quarter, but with the iPhone 12 launching almost a month late, that will likely impact the number Apple can sell. The iPad Air (2020) will probably sell a few million during this period, as well. I can't see the first Apple Silicon laptops selling more than a handful of million during the quarter so it's possible TSMC will have the production capacity to meet all the demand, but it depends on how many A14 SoCs TSMC has delivered to "prime the pump" in terms of pre-sale date production of the iPhone 12, iPad Air (2020) and AS Mac (if it is using the A14).
Throughout 2021, TSMC is expected to double their 5nm production as well as move to the 2nd generation of their process. I expect these will be the basis for the high-performance Mac SoCs for the 16" MBP and 29-32" iMac (and iMac Pro?) that will likely ship in mid-to-late 2021
A12X was released just one month after A12 in Oct 2018. Two weeks after A12X Apple released iPad Pro with A12X. They could still do a similar thing with A14X and iPad Pro. However A12Z wasn't released until six months later in March 2019 so we may have to wait for A14Z until early 2021.
The A12X and A12Z both have 8 GPU cores (you can see the blank area in the GPU rows on the A12X that is filled on the A12Z), but with the A12X, the defect rate on one of those GPUs was likely high enough that Apple chose to just enable 7. As the yields improved, the defect rate dropped to the point that most of the SoCs had 8 good GPUs so Apple enabled them all and sold them as the A12Z.
I believe Apple will add MiniLED to the next iPad Pro as another point of differentiation from the iPad Air (2020) and I feel that will not happen until Q1 2021 based on reports of MiniLED controller production. That should hopefully allow the A14X to launch with all of the planned GPU cores enabled for the iPad Pro (2021) and the "A14M" for the 2021 MacBook Pros and iMacs.