we all can not tell you, how many stitches you have to cast on. That majorly depends on how thick your yarn is, what needles you use and how tight you knit.
to find out about this there is the “trick” people told you above:
you cast on some stitches. Use the yarn and needles you want to work with. I would like 20 stitches for a sample like this (do 2 side stitches (just knit for example) one on each side - so you can do your pattern well (two times nine + 2 side stitches). Then you knit in your pattern for a bit. Do not cast off or cut the yarn. you will rip this out again (I tend to do that).
Now take a measuring tape. lay your sample flat, don’t stretch it.
now measure (not from the edge but maybe 2 stitches in) and count the stitches that you see within 1 inch above your tape measure.
the count comes out much better if you measure something like 4 inches and count those stitches. Less “fractional” stitches.
Now you know how many stitches you make per inch. (if you measured and counted for more than 1 inch: devide the number of stitches by the number of inches)
you know how many inches you want your blanket wide: 30 inches.
now you also know how many stitches you have per inch, lets say: 10.
then you multiply 30 inches with 10 stitches/inch and end up with 300 stitches. Now that would make 33 and 1/3 pattern repeats, if you do no border to the blanket (advisable to have a border, really)
so you find out how many stitches you want, really. if you do 297 then you have 33 repeats. if you do 306 you have 34 repeats and so on.
But THESE numbers are just an example. You need you swatch first and you need to do the math yourself (or have us help).
if you had 15 stitches to the inch you would have 450 stitches total, if you had 8 stitches to the inch only you would have 240 stitches. it really depends.
for the length of that blanket you CAN do the same: measure your swatch (not the stitches on the needle or the cast on or a cast off) and multiply. But if you want to just knit the thing in the same pattern that is not necessary. You can just keep on knitting and checking the size. When you are getting close you can work the last repeat (and maybe a borderline) and finish.
ONE important advice: when you do the math and start… take a break after a few rows of such a big project and check your total. You would not be the first to go very wrong because of a mistake in multiplication or counting. If you come out approx. right: go ahead.
with a project like this and being tied to repeats of 9 stitches you will never get the sice to the milimeter right. But you won’t need to with a blanket, right?
Have fun with your big projects!