IIRC the biggest Zen3 mobile CPUs are 8-core. So they'll have at most 2x cores. And that's ignoring the low-power cores on the mac which probably still count for half a core each.
AMD is likely to be faster in multicore overall, but not by much it seems.
There are no announced or released Zen 3 mobile CPUs at this time. You are correct in that the Zen 2 mobile CPUs currently top out at 8 cores, and up to about 54W TDP - the top CPU is the "45W" Ryzen 9 4900H which can be configured up to about 54W by the OEM. We might see Zen 3 mobile early in 2021.
AMD is likely to be faster in multicore overall, but not by much it seems.