Afaik they're not doing 20 GB cards with this launch, but rather it's more likely next year.
Is it enough? As always - it depends. On the game. On the settings. On the scene (eg Cities see sky-high vram usage in Flight Sim, but other areas not so much). On the action (how fast you're going through new scenery & therefore how much you can hold in memory & swap between) etc.
I know for a fact that 8 GB is not enough at 4K. Not in all titles, and not in all titles is it a big deal (eg freezing rather than more common stutters), but in plenty. Doom Eternal? Murders 8 GB. The Wolfenstein games with streaming turned up? Same. COD? Sure. Greedfall? Yup. The Division 2? Stutters for sure. Far Cry 5 w/ HDTP? Absolutely. MSFS 2020? We already know. Shadow of War? Worse off. Skyrim mods? You betcha. VR? Oh boy. And so on, and so forth.
Now, in those cases 10 GB would be enough but it would skirt right at the edge of going into 'let's see some stutters' territory. For new games going forward? Who knows. If they're RTX-IO/DirectStorage optimised, I'm hopeful, but that alone won't solve the need for more vram, just alleviate it. DLSS would also be a massive help but again, not something you can rely on to always be there. Like id Engine's lead programmer says: 8 GB is the new MINIMUM. You wish to turn up everything & enjoy? Better pucker up because 10 GB ain't worth jack.
Fundamentally I'm okay with 10 GB overall for 2 years. I would absolutely not be okay with it if I intended to keep it for longer. And knowing there are cards with 16 GB/20 GB coming out in the next few months? Really hard to stomach buying a 10 GB card at the high-end for the long haul. If I weren't currently stuck with a 480 I'd probably be more inclined to wait. I really think AMD's gonna make Nvidia work for their cut this year.