Source

File: wp-includes/ms-functions.php

function wp_update_network_user_counts( $network_id = null ) {
	global $wpdb;

	$count = $wpdb->get_var( "SELECT COUNT(ID) as c FROM $wpdb->users WHERE spam = '0' AND deleted = '0'" );
	update_network_option( $network_id, 'user_count', $count );
}