Might not be a good option though if you also spawn a bunch of blocking io tasks, which benefit from a large number of threads.

Exactly. I do not know the specifics, but for example if libraries you call into liberally spawn_blocking under the expectation that it is okay, you will be in trouble.

Says it right there actually:

> It’s recommended to not set this limit too low in order to avoid hanging on operations requiring spawn_blocking.

So a total like 6 - reasonable for a web backend - would be way too low.