Ahh ok my apologies! We have a Cloud Platform feature that works something like that called
Task Concurrency limiting; this feature works by "tagging" a task and providing a global concurrency limit. Prefect then ensures that no more than the limit of any given task with that tag is run at any given time.
However I'm not sure that exactly fits your use case, which if I understand correctly is retrying mapped tasks that fail without having to retry every task in the mapped list. In that case, you can set the max retries and retry delay directly and each failed mapped task will retry on its own (without impacting the other tasks in the set).
More details on that here.
Let me know if that's a little closer to what you were thinking!