If you mean Ryzen 9 5950X, which is the top of the consumer/retail line, the top board is the
Crosshair VIII Extreme. N.B. it's an EATX board which can be an issue for some cases; if you want ATX, then Crosshair VIII (Dark) Hero or Formula, Formula for custom water loop.
AMD made the 5000-series Threadrippers OEM-only, not for retail sale, although I believe you can get them if you really want them and have the $$$. The normal 5950X is like a mini-Threadripper and sufficient for most power users, but if you have pro money rather than enthusiast money, that's up to you. If you really need 256GB RAM, you'll need to go Threadripper, as AM4 5000 series is 128GB max. Threadripper would be on the Zenith line of boards, but I don't know if any support Threadripper 5000s due to the not-for-retail thing.
You might want to look into AM5 and the Ryzen 7000 series, as they are both imminent. ASUS have already announced the
ROG Crosshair X670E Extreme, which will likely be their top AM5 board. I'm not sure if AM5 / X670 platform has the same 128GB limit as AM4 / X570, as the tech specs are still difficult to come by and in the pre-release stage.
Without knowing your exact plans and needs, I'd advise going for 5950X or 7950X (non-Threadripper, AM4/X570 or AM5/X670) unless you really have a good reason to go Threadripper pro. You can easily spend north of $10k at the top end of X570/670 platform, on a full system, if you want to.
