You can look on various sites that track achievements to see the numbers. Only about 6% of players have cleared Ultimates, so they’re definitely a small minority of the playerbade.

Interestingly, by comparison, only about 0.1% of players have solo’ed PotD. This means that there’s a lot of folks who want semi-difficult content (Ultimates) but are not good enough / do not have the patience to do actually difficult content (soloing the deep dungeons). And it’s hilarious how salty they are.