I rarely run 10 top-level sessions, but I often run multiple.

Here is one case, though:

I have a prototype Ruby compiler that long languished because I didn't have time. I recently picked up work on it again with Claude Code.

There are literally thousands of unimplemented methods in the standard library. While that has not been my focus so far, my next step for it is to make Claude work on implementing missing methods in 10+ sessions in parallel, because why not? While there are some inter-dependencies (e.g. code that would at least be better with more of the methods of the lowest level core classes already in place), a huge proportion are mostly independent.

In this case the rubyspec test suite is there to verify compliance. On top of that I have my own tests (does the compiler still compile itself, and does the selftests still run when compiled with self-compiled compiler?) so having 10+ sessions "pick off" missing pieces, make an attempt see if it can make it pass, and move on, works well.

My main limitation is that I have already once run into the weekly limits of my (most expensive) Claude Max subscription, and I need it for other things too for client work and I'm not willing to pay-per-token for the API use for that project since it's not immediately giving me a return.

(And yes, they're "slow" - but faster than me; if they were fast enough, then sure, it'd be nicer to have them run serially, the same way if you had time it's easier to get cohesive work if a single developer does all the work on a project instead of having a team try to coordinate)