How many Nginx processes should there be?

This is the number of your worker processes of nginx. The default configuration has 4 worker processes. They are here that the web server can handle multiple connections very fast. One of them is the master process see the output of ps fax | grep nginx:

 user@host:~# ps fax | grep nginx
 6885 ?        Ss     0:00 nginx: master process /usr/sbin/nginx
 6886 ?        S      0:00  \_ nginx: worker process
 6887 ?        S      0:00  \_ nginx: worker process
 6888 ?        S      0:00  \_ nginx: worker process
 6889 ?        S      0:00  \_ nginx: worker process

You see there is a master process and 4 child processes (workers).

Ot determine how many worker processes are configured type:

user@host:~# grep worker_processes /etc/nginx/nginx.conf
worker_processes 4;