Get some desired rows from table by its order

You can already fix your code with sql queries (for faster performance) adding LIMIT 20, 10 (get 10 records beginning with row 21)

but if you have other logic to be applied on your code (as if your displaying dynamic records)

$row=mysqli_fetch_assoc($result);
for($num=0; $num<=$result->num_rows; $num++) {
    if($num>=21 && $num<=30){
        echo "<tr>";
        echo "<td>".$num."</td>";
        echo "<td>".$row['email']."</td>";
        echo "<td>".$row['name']."</td>";
        echo "<td>".$row['description']."</td>";
        echo "</tr>";
    }
}

other thing you might consider is using foreach

$row=mysqli_fetch_assoc($result);
$num = 0;
foreach (range(21, 30) as $row) {
    $num++;
    echo "<tr>";
    echo "<td>".$num."</td>";
    echo "<td>".$row['email']."</td>";
    echo "<td>".$row['name']."</td>";
    echo "<td>".$row['description']."</td>";
    echo "</tr>";
}

Please, try with sql query, where you defined limit of rows like start from row no to end of row no. I wrote one sql query at bellow:

Syntax:

SELECT column1, column2, ...
FROM table_name
LIMIT offset, count;

Example:

 SELECT * from MYTABLE LIMIT 20,10;

Two possibilites you can doing this way

First

SELECT * from MYTABLE LIMIT 20, 10

Second

// 0,1,2,3....19 19 is record of 20
for($num=19;$num<=29;$num++){
 }

You may try like below.

SELECT * FROM Tablename WHERE id BETWEEN 21 AND 30

Where, id -> Will be the unique key of the table.

Thanks.