Yeah, I agree you should test the breaking strength if the linen doesn't already state what the breaking strength is. Than, once you figure out the breaking strength, (say it is 20 lbs), add enough strands to account for the draw weight of the bow x 4 or more, I usually do about 4 to 6 times the draw weight. So, for example, if your strands are 20 lbs breaking strength, and your bow is 80 lbs... you want enough strands to equal 320 lbs or more for 4 times the draw weight, in this example 16 strands since the strands would be 20 lbs breaking strength.
It's best to check and be sure, not all linen is equal, like the stuff you get at the sewing store in little packs can be way less quality than the good stuff (which is the stuff you want)