Skylake-X/-SP exists in three die sizes; 10, 18 and 28 cores. If it's true Intel can fit >18 cores on LGA 2066, then they can fit the full 28. If not, 18 will be the maximum.
Currently Intel have these high-end platforms:
Consumer HEDT: (X299): 10 and 18 core dies
Xeon W (C422): 10 and 18 core dies
Xeon Scalable (C621): 10, 18 and 28 core dies
The dies themselves are the same, they are compatible with all of these. Rumors claim Intel will end up with the consumer HEDT spread across two sockets, which is completely unnecessary.