With a backend (like php) you are always bound to what the backend can handle, nginx is just a portal here.
The amount of backends should be balanced with the best balance setting like leastconn/iphash, ea: http://nginx.org/en/docs/http/load_balancing.html and also consider Lua for managing/offloading backends.
So its not really a number game but a distribution one.

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nginx for Windows http://nginx-win.ecsds.eu/