At this point you can get 24TB of RAM in an EC2 instance (along with 448 vCPUs, 100Gbps of network bandwidth and 38Gbps of EBS bandwidth). That won't scale forever, but Stack Overflow has been running on a single primary/standby setup with 1.5TB of RAM so that would be 16x Stack Overflow's RAM.
I think a lot of work goes into horizontal scaling which is necessary at a certain scale, but very few people actually get anywhere near that scale. It can be important to understand which things are needed at your scale and where you can simply buy some beefier hardware. I've been at places where people run a dozen sharded DB servers with each server having 16GB of RAM. Maybe that's resume-driven-development where someone wants to say they've done that.