There are some tools that make horizontal sharding much easier, like Vitess which is MySQL only. But most businesses won't come close to needing these kinds of capabilities.

I would say for anyone starting out and worrying about how large a single node can scale, Postgres can run well on aa 64/128core server, 2TB ram and 20TB ZRAID6 with a chain of read replicas. This can be done out of the box on Postgres without much issue and can get many businesses quite far, but once you go to the lots and lots of TB, or you have specific write latency, consistency, or other requirements, you have to evaluate multiple databases against your company's specific usage patterns and data, as no benchmark will give you a good idea.