如何限制kotlin协程的最大并发性

I've got a Sequence (from File.walkTopDown) and I need to run a long-running operation on each of them. I'd like to use Kotlin best practices / coroutines, but I either get no parallelism, or way too much parallelism and hit a "too many open files" IO error.

File("/Users/me/Pictures/").walkTopDown()
    .onFail { file, ex -> println("ERROR: $file caused $ex") }
    .filter { ... only big images... }
    .map { file ->
        async { // I *think* I want async and not "launch"...
            ImageProcessor.fromFile(file)
        }
    }

This doesn't seem to run it in parallel, and my multi-core CPU never goes above 1 CPU's worth. Is there a way with coroutines to run "NumberOfCores parallel operations" worth of Deferred jobs?

I looked at Multithreading using Kotlin Coroutines which first creates ALL the jobs then joins them, but that means completing the Sequence/file tree walk completly bfore the heavy processing join step, and that seems... iffy! Splitting it into a collect and a process step means the collection could run way ahead of the processing.

val jobs = ... the Sequence above...
    .toSet()
println("Found ${jobs.size}")
jobs.forEach { it.await() }