One option is to use an SQL statement but WordPress provides a nice function called count_users.
When you call this function it will output a result like this.
Result
[code]
	array (
	  'total_users' => 6,
	  'avail_roles' =>
	  array (
		'administrator' => 2,
		'editor' => 2,
		'subscriber' => 2,
	  ),
	)
[/code]
Here is how to use it.
[code]
	$result = count_users();
	$total_cnt = empty( $result[ 'total_users' ] ) ? 0 : $result[ 'total_users' ];
	echo 'Count: ' . $total_cnt;
[/code]
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