Sorry I didn't answered previously about the number of local machines. Actually it depends much more on the network configuration than the number of machines itself in our case.
We used two machines to simulate up to 600 requests per second, that was much more than our average use, actually much much more. And we measured it without HAProxy or Varnish or similar systems between our servers and the clients.
We used two machines to simulate up to 600 requests per second, that was much more than our average use, actually much much more. And we measured it without HAProxy or Varnish or similar systems between our servers and the clients.