The number of pairs of jeans you need for your quilt really depends on size you wish the blanket to be. This time, I am making a smaller lap blanket that is roughly 4.5 ft by 5.5 ft. I was able to complete this blanket with three pairs of adult jeans.
The first step to readying your jeans is opening them all up. The easiest way to do this is to cut out the middle seam that runs down the crotch. This seam includes the zipper in the front and then runs down and around to the back, right between the two back pockets. From the second and third image, you can see how I cut right now one side of the seam, all the way around, until the two sides (left leg and right leg) are separated. Once this first cut is made, simply cut the seam off of the other side. Now, open up the two legs by cutting out the seam that runs down the inside of the leg. You can see this in the final image.
If at any time in this process you find a particular part of a pair of jeans you wish to use in your quilt, it is quite easy to improvise. Each quilt will be incredibly unique and each piece can have special memories and meaning. I really enjoy pockets and try to save any pieces with hammer loops to use for my quilts.