By default the number of packets are set to 5. Anyhow, you can change that to whatever youwish in check_ping command definition by adding the -p switch.
Try to run the check_ping plugin from the terminal:
/path/to/check_plugin --help
and it will show you all of the options. Try to execute it from the terminal then with the regular host and see what response do you get.
5 is ok to determine the ping reachability of the host.
If you have continuos packet loss then it could be related to your nagios server and it’s overload or heavy network traffic.