There are a total of 38 numbered seats in Section 506 Row 38. As you face the field from the section, seat 1 will be on the aisle to the right, while seat 38 is on the aisle to the left.

Therefore, seats 19-21 will be almost in the middle of the row, but are more easily accessed via the aisle to the left (between sections 505 and 506) as you will have to pass 17 other seats, instead of passing 18 other seats from the aisle on the right.