Stumped on direction for a shrug, Help!

I am knitting the Gray Ghost shrug by Blue Alvarez Designs and after spending 2 days trying to figure out what this means I have realized I need help from experienced knitters! :cheering:

I included the steps leading up to where I am confused because they figure in to my confusion!

"Work 10[12,12,16] rows On the next row (WS), work the following short row segment, L and XL:
WS(1): P12, w&t.
RS(2): K12
WS(3): P16 (picking up wrapped 12th stitch as you knit it), w&t.
RS(4): K16
WS(5): P20 (picking up wrapped 16th stitch as you knit it), w&t.
RS(6): K20

Work 18 full rows in, [COLOR=“Red”]Pause to tie a piece of scrap yarn into the beginning stitch of any existing WS row[/COLOR]. This will serve as a reference later."

I know this is to differentiate the sides later on when I complete the body but does it mean to put scrap yarn at the beginning of every WS row starting at the beginning (for the back/shoulders)? Ex: For size L there would be 18 WS rows?

No. You just need one piece to differentiate between the two edges.