Nothing much, you need to add more backend servers to handle more requests, for example php-cgi is a single thread backend, if requests are handled fast one will suffice if not you add more backends. If 1 request takes 10 seconds you need 10 backends to handle that load for a 10 users, etc.
Or get the backend to handle requests via a pipe asynchronously, tcp handling has its limits.
Or get the backend to handle requests via a pipe asynchronously, tcp handling has its limits.