How to fire N Requests per Second in Scala - multithreading

Question

I am developing an application in Scala, which is kind of request processor in batch. It gets the required data from storage and forms the request and calls an API of another service (named ServiceB)
ServiceB allocates 110TPS of its throughput to my service. In order to utilize max available throughput, I want to make request at the rate of 100TPS.
How can I fire requests at rate of exactly 100 TPS ?
In theory, if the API call takes around 500 milliseconds to execute. One thread can execute max 2 request per second. So, 50 threads are needed to achieve 100TPS per second.
But Futures in scala are anyway executed parallely. This makes it difficult to come up with an exact number of threads required.
I tried the following
def main(args: Array[String]): Unit = {
val executorService = Executors.newFixedThreadPool(1) // threadpoolsize 1
implicit val executionContext: ExecutionContextExecutor = ExecutionContextFactory.get(executorService)
....
val request = getRequest(..)
val responseList = ListBuffer[Future[Response]]();
val stTime = System.currentTimeMillis()
val rateLimiter = RateLimiter.create(300) //guava-ratelimiter
while(System.currentTimeMillis() - stTime <= (1000*60*3)) { // Running for 3 mins
rateLimiter.acquire(1)
responseList += http(request) // using dispatch
}
//TODO : use different threadpool to process the futures in responseList.
}
Metrics from DataDog:
TPS is 83
Total no of calls to tat api is 16.4k
This doesn't seem to be firing calls at even 100 TPS (though I allowed 300 request per seconds, check the rate limiter value)
This problem is similar to load testing, how are load testing frameworks firing requests at X TPS constantly?
Thanks.