Cut 9 strands of paracord twice the length you want your reins to be. Measure and cut one cord first, then use it as a guide for cutting the others. You can use however many colors you want, but keep the color combination in mind. If you want a thinner set of reins, cut 6 strands instead.
9 to 12 feet
The general rule for the cobra weave, and many other weaves, is: “One inch of bracelet equals one foot of paracord.” This is a generally accurate rule, unless you have very large wrists, you are making a wide bracelet, or you are using smaller diameter cord.