Limit Http Client throughput

Hi! I want to ask if it possible to limit by, say, 50 concurrent outbound requests using Akka Http Client API?
I tried with Source.queue[(HttpRequest, Promise[HttpResponse])] and Http().cachedHostConnectionPool but external API which I’m calling return 429 error with retry-after and I don’t want to hit that.
My code

val poolClientFlow
    : Flow[(HttpRequest, Promise[HttpResponse]), (Try[HttpResponse], Promise[HttpResponse]), Http.HostConnectionPool] =
    Http().cachedHostConnectionPoolHttps[Promise[HttpResponse]](host = host)

val queue: SourceQueueWithComplete[(HttpRequest, Promise[HttpResponse])] =
      .queue[(HttpRequest, Promise[HttpResponse])](bufferSize, OverflowStrategy.backpressure)
      .throttle(elements = maxNumberOfRequests, per = 1 minute)
      .groupedWithin(50,1 second)
        case ((Success(resp), p)) =>
        case ((Failure(e), p)) =>

def send(request:HttpRequest) = {
    val responsePromise = Promise[HttpResponse]()
    queue.offer(request -> responsePromise).flatMap{...}

I also tried using global limit Actor described here : but no success. My idea here was to release token after Future completes.

.throttle() is not helping too (

Unfortunately I cannot rely on max-open-connections because my server can has multiple clients. And each client should be able to do 50 concurrent requests with Akka Http Client API