According to this documentation, each trace is stored in the object as an array see print_r ($db->trace); at the end.
Query exectution time benchmarking
To track query execution time setTrace() function should be called.
$db->setTrace (true);
// As a second parameter it is possible to define prefix of the path which should be striped from filename
// $db->setTrace (true, $_SERVER['SERVER_ROOT']);
$db->get("users");
$db->get("test");
print_r ($db->trace);
    [0] => Array
        (
            [0] => SELECT  * FROM t_users ORDER BY `id` ASC
            [1] => 0.0010669231414795
            [2] => MysqliDb->get() >>  file "/avb/work/PHP-MySQLi-Database-Class/tests.php" line #151
        )
    [1] => Array
        (
            [0] => SELECT  * FROM t_test
            [1] => 0.00069189071655273
            [2] => MysqliDb->get() >>  file "/avb/work/PHP-MySQLi-Database-Class/tests.php" line #152
        )
So you can get the number of queries executed by just doing 
$num = count($db->trace);