The BOM can be confusing, so believe the shopping list. The BOM is an indented "tree". You will see that (2) are used on a z lead screw, but there are multiple z lead screws. You need to multiply the quantity by the quantity of the parent assembly and so on, recursively all the way to the top. Welcome to manufacturing
BTW: About the last dozen kits sent and all future kits have all of the 1/4" ID bearings included. They are flange bearings pressed in now. I have not changed the BOM yet, because I did not want anyone caught between changes. The change was more for my productivity. It makes the mating plastic parts simpler. The holes used to secure the bearings are a PITA because it is only a plunge and causes plastic to creep up the bit. I also can remove the cbore operation.