title: Limit number of requests at a time with RxJS
body: <p>Suppose I have 10 urls, and I wish to make an HTTP request for each one of them.</p>
  
  <p>I could create an observable of the URLs, then <code>.flatMap()</code> the requests for each one of them, and then <code>.subscribe</code> for the results. But that would make all of the requests at once.</p>
  
  <p>Is there a way to impose a limit to the number of requests to a fixed number, in order not to overload the server</p>