Maybe 256 so it doesn't need changing again anytime soon.

Yea just wait until Threadripper 3...

To build on this, I have a working kernel using 4.18-rc8 on my laptop, haven't rebased patches against 4.18 proper but I'll do that and make sure said patches get applied when we moved to 4.18 (presumably .1). I have an AMDI0011 controller patch in there (enables AMD MP2) which I need to even get close to maybe getting my Elantech touchpad working (still something I'm actively working on getting resolved).

1. I'd like to be conservative and set it to 64. That will cover us for another year without penalizing low-core count systems too hard (64 cores -> 512KB added to the image for everyone, regardless of cpu count).

This allows you to specify the maximum number of CPUs which this kernel will support. If CPUMASK_OFFSTACK is enabled, the maximum supported value is 8192, otherwise the maximum value is 512. The minimum value which makes sense is 2.
This is purely to save memory: each supported CPU adds about 8KB to the kernel image.

and

This is purely to save memory - each supported CPU adds approximately eight kilobytes to the kernel image. For best performance should round up your number of processors to the next power of two.

2. Can do.
3. Yeah, not needed.

Not a kernel config, but would be good to land D3597 for next kernel update