Syntax: worker_connections number
Default:
The worker_connections and worker_processes from the main section allows you to calculate max clients you can handle:
max clients = worker_processes * worker_connections
In a reverse proxy situation, max clients becomes
max clients = worker_processes * worker_connections/4
Since a browser opens 2 connections by default to a server and nginx uses the fds (file descriptors) from the same pool to connect to the upstream backend