The multiple thing----most patterns are worked over some number multiplied. Like K1, P1 ribbing is any multiple of 2. You can work it over 2,4,6,8,10,12, etc. stitches. And K2, P2 ribbing is a multiple of 4. It can be worked over 4,8,12,16,20,24,28 etc. And some patterns are worked over a certain multiple but the way they have written the pattern you have to add another number of stitches for everything to come out right. In this case any multiple of 2 with one more added. Or it could be like a multiple of 8+3 so then it would be 8+3, 16+3, 24+3, 32+3 and so on.
I'm with you on everything but the last sentence.
When you said that did you mean you agree with everything I said but the last sentence or you understand everything but the last sentence?
Assuming the latter.....
The pattern comes out like this:
v n v n v n v row 4
v v v v v v v row 3
v v v v v v v row 2
n v n v n v n row 1
I'm hoping these are lined up with each other an 'n' is a purl stitch, and a 'v' is a knit stitch. You start with the bottom row and knit upward. The first stitch in the first row is a purl (n) and then they alternate across. Then you have two rows of stockinette stitch that all look like knits on the right side. On the next row you work it the same way you did the 1st row, starting with a purl, but you do it from the wrong side so that on the front it looks like you started with a knit (v) and worked alternately across.
So the next time you need to do an alternating row (row 3 or 5) look at the last one and do them the opposite. So if it the last row was:
n v n v n v
You want it to end up
(on the right side)with knits over purls and purls over knits from the last row you did with the bumps. So you want knits above the bumps (the purls) and purls above the knits (the smooth vees). So just study your knitting and make the stitches line up the way they need to on the right side.
You probably already did a back and front of this sweater so you should know what it looks like, make it look like that. Any slight wobble in the pattern will be right at the underarm seam and won't show.