Just go to to the developer tools -> Template and put the below code in there. Sum the total and use that in the poll (feel free to copy paste the result in the comments :-))

Sensors: {{ states.sensor | count }}
Lights: {{ states.light | count }}
Switches: {{ states.switch | count }}
Binary sensors: {{ states.binary_sensor | count }}
Automations: {{ states.automation | count }}

Source: How much entities do you have in Homeassistant? – Home Assistant Community