The NeXT variant did run on the following architectures:

1. Motorola 68k (the original NeXT hardware had 68030 and 68040 chips)

2. Intel x86 (NeXTSTEP 3.1 for Intel was released in 1993)

3. HP PA-RISC (I have an OPENSTEP 4.2 CD that can run on Motorola 68k, x86, PA-RISC, and SPARC hardware)

4. Sun SPARC

5. 32-bit PowerPC (Rhapsody, the original Mac OS X 1.0 that was essentially still Rhapsody, and of course Mac OS X from Cheetah through Leopard)

6. 64-bit PowerPC (Power Mac G5 and iMac G5)

7. Intel x86-64 (starting from Mac OS X Tiger all the way to macOS Tahoe)

8. 32-bit ARM (iOS on early iPhones with 32-bit ARM chips)

9. 64-bit ARM

I could be forgetting other platforms, but these are the ones I know from the top of my head.

Indeed—I meant specifically the NeXT branch of the family tree because of this exhaustingly long list.

I would very much like to see that quad-fat OS4.2 CD; most NeXT releases around that era drop PA-RISC and are only tri-fat. I only have a 3.3 RISC (HPPA+SPARC) ISO for HPPA coverage.

The big ones you're missing are the Intel i860 (used as a graphics accelerator on NeXTdimension video processing boards—also the original target platform for the Win NT kernel) and the Motorola 88k family, which was briefly explored for the "NeXT RISC machine" in the mid-90s; only one prototype is known to exist. There were non-NeXT ports of Mach to m88k, which may have influenced the decision.

Of course, if we add in the other branches of the Mach family the number of ports gets absurd! It originated on the VAX; OSF/1 adds MIPS and AXP to the list... ultimately RISC-V and Itanium are the only significant ≥32-bit CPUs of the last forty years to not see some kind of Mach port.

But—the ultimate point is that the lion's share of actual work porting the kernel to new hardware is thanks to NeXT and/or NeXT cosplaying as Apple.