how to find a suitable number for the keepalive connections ?

[mozatkey]

The |/connections/| parameter should be set low enough(??)
to allow upstream servers to process additional new incoming
connections as well.


I am very confused about the keepalive <http://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive> connections in the ngx_http_upstream_module, 
can somebody  tell me how to determine the keepalive connections in http protocol ?


just like this:
upstream http_backend {
    server 127.0.0.1:8080;

    keepalive 16;// why 16 ? how to determine a suitable number ?
}

server {
    ...

    location /http/ {
        proxy_pass http://http_backend;
        proxy_http_version 1.1;
        proxy_set_header Connection "";
        ...
    }
}

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