Do I need to restart zabbix agent or zabbix server after creating new item in an existing Template?
Just want to know because I want to minimize the impact as it's in production.
If you want the checks to apply immediately you must refresh the configuration cache of the Zabbix server|proxy responsible for monitoring the agent.
This is done with
zabbix-server -R config_cache_reload
or
zabbix-proxy -R config_cache_reload
To Answer your question -- No
The agent will receive it's next set of checks from the proxy / server on it's next config cache reload, at default this is rather quickly but can be done imediately with the above shell commands.
There are situations where you will need to restart the agent though.
If you've created a new userparameter for the agent then the agent will not recognise this key until the agent is restarted.
No, not needed to restart the agent. You only need to restart an agent if the config file[s] change. Zabbix server even less.