13.1Start with Cmaj7

C major scale:

C D E F G A B

Cmaj7:

C E G B

Now continue stacking thirds:

add D:

C E G B D

That is:

Cmaj9

Add F:

C E G B D F

That gives the 11th.

Add A:

C E G B D F A

Now we have theoretically reached:

Cmaj13

So the full stack is:

1 3 5 7 9 11 13

For C:

C E G B D F A

That is all seven notes of the scale arranged in thirds.