The pattern as written is a total of 342 rows (19 groups of 18 rows). You need approximately enough yarn to work 5 sets of 18 rows per color:

White: 92 rows ((5 x 18) + 2)
Light blue: 90 rows (5 x 18)
Dark blue: 88 rows ((4 x 18) + 16)
Tan: 72 rows (4 x 18)

So if you want to change the gauge or width, keeping track of how much yarn it takes to knit the cast-on and first 17 rows will be approximately 1/5th of the amount needed for each color.