How do I only add items up until the concurrency limit and then wait until a pending promise is fulfilled?

[WayneEllery]

How do I only add items up until the concurrency limit and then wait until a pending promise is fulfilled? I couldn't work out how to do this? When I use .onSizeLessThan(1) it will wait when there is same number items as concurrency limit + 1 but I want to stop adding items when it hits the concurrency limit