In the database posts & discussions here I sometimes find that the opinions are strong but the numbers are missing. Obviously applications have wide variation in traffic and query complexity so apples to apples comparisons are hard. Still, I would greatly benefit from hearing some real world experiences with numbers.
Rough approximation database questions for current or prior applications:
1. How many customers do you have?
2. What's expected daily traffic? Peak traffic?
3. What database engine or engines do you use?
4. How many rows or how much storage does your db have?
5. What else about your application is relevant for database load?
6. Microservice, Service, or monolith. Happy with it?
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.