Napkin math is how you avoid spending several weeks of your life going down ultimately futile rabbit holes. Yes, it's approximations, often very coarse ones, but done right they do work.

Your question about what degree of parallelization is unfortunately too vague to really answer. SSDs offer some internal parallelism. Need more parallelism / IOPS? You can stick a lot more SSDs on your machine. Need many machines worth of SSDs? Disaggregate them, but now you need to think about your network bandwidth, NICs, cross-machine latency, and fault-tolerance.

The best engineers I've seen are usually excellent at napkin math.