Update 2/14/18 4:45 PM: Intel confirmed to me via email that its specifications regarding these chips are accurate. The original article continues below.
Despite initial word from the horse's mouth that Intel's Core i7 Skylake-X CPUs would boast only one AVX-512 FMA unit per core and Core i9 CPUs would have two such functional units per core, official documentation pointed out by CPU detective InstLatX64 on Twitter today confirms that Skylake-X Core i7s and Core i9s are actually equally capable in that regard. Intel's own product comparison page, when loaded up with the Skylake-X family, shows that every chip of the brood is specced with two AVX-512 FMA units—a figure the company confirmed is correct.
Although we never got around to putting Intel's specs to the test, thanks to questions regarding the capabilities of engineering samples versus production chips and a dearth of benchmarks actually capable of employing AVX-512 instructions, some forum members at HWBot did test the Core i7-7800X versus the Core i9-7900X using the Flops tool last year. As its name suggests, Flops simply extracts the highest number of floating-point operations per second from CPUs, including chips with AVX-512 support. Those users saw similar per-core AVX-512 throughput from the i7-7800X and the i9-7900X, suggesting that the chips did in fact have two functional AVX-512 FMA units per core.
A block diagram of the Skylake Server core
AVX-512 is still in its infancy for desktop users and may be quite some time from widespread adoption, but folks looking to program for the new instruction set would seem to be able to enjoy full potential AVX-512 throughput from any Skylake-X product, not just the pricey Core i9 line. Those capabilities would mesh with Intel's Xeon W family of chips, all of which are built from the same Skylake Server silicon and have two AVX-512 FMAs enabled per core. If I can find the time soon, I'll put our own Skylake-X CPUs to the test with Flops and report what I find.