How to find a suitable number for the keepalive connections?

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 "";
    ...
}

}