Hi Kishor,
We don't have any API currently to fetch the count of the available Users in one's Org.
However, we've the /list-people API endpoint which result you can store in a variable.
Once the details stored into the variable, you can use the JSON loads() function to convert it into a Python object or, Dictionary and use the len() function to find its length.
That way, you can fetch the count of the dictionary which should give you the length of Users calculated manually.
You can also try out some other length calculation options manually using other coding styles.
Please let us know for any further assistance!