WordPress at a glance
function is not described

get_user_count() WP 3.0.0

The number of active users in your installation.

The count is cached and updated twice daily. This is not a live count.

No Hooks.

Return

Int. Number of active users on the network.

Usage

get_user_count( $network_id );
$network_id(int|null)
ID of the network.
Default: current network

Changelog

Since 3.0.0 Introduced.
Since 4.8.0 The $network_id parameter has been added.

Code of get_user_count() WP 5.7

<?php
function get_user_count( $network_id = null ) {
	return get_network_option( $network_id, 'user_count' );
}