I think you might be asking the wrong questions. They key questions are Queries per second and the median response size of the query.
For example at reddit (15 years ago) we had 10x more vote traffic than comment traffic, but we only needed two databases to handle votes (technically only one the other was just for redundancy).
But we needed nine comments databases. Mainly because the median query response was so much bigger for comments.
The users of our apps are pretty tolerant of 5 to 10 minute request times for some of our pages, which means we've been able to get away with just a few servers for several TBs of data stored and served. (100+mb responses are not unusual for us).
If we had to rethink and redesign the system to cut down those times, we'd need a lot more databases and a much cleverer storage strategy than we currently have.
While I'm sure response time for Reddit is really important, I could imagine that an IOT serving system needs almost nothing in to hit something like a 10 to 20 second response time for 5k devices.
For example at reddit (15 years ago) we had 10x more vote traffic than comment traffic, but we only needed two databases to handle votes (technically only one the other was just for redundancy).
But we needed nine comments databases. Mainly because the median query response was so much bigger for comments.