Unless we have a strong reason to keep them coupled, it is awkward to have the number of collections we dump in parallel be tied to the hidden --numThreads flag. NumThreads internally tunes the Go runtime to multiplex goroutines across N operating system threads. This is orthogonal to dumping a collection, which is almost entirely I/O bound.

As a naming suggestion, I think it would make most sense to mimic the j / numParallelCollections naming from mongorestore.

Even if we still want to leave the flag undocumented/hidden, we should decouple it from numThreads.