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
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 {

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 "";