I also think the Ibex were hunted almost to extinction for the horn to make crossbows from, but you should need only one or at most two horns to build a composite bow, the horn is sawn into many narrow strips no more than between 5 and 8mm thick and 12mm wide, and even though you cannot use much of the side pieces, the front and back can be used and should supply plenty of material. You can use strips as short as 15cm, perhaps even shorter, I do not think the medieval builders wasted any material, even building bows just out of scraps but double layered to compensate, though this is just my theory.