According to this, Amazon has about 170 million items on its catalog. About 135 million items are physical products. For Apple, they have about 26,000 items (rough estimate, subject to change). The point is that, if you have to make a demand forecast, which one will more difficult for you, 135 million items or 26k items.
Product Life Cycle
Put it simple way, the product life cycle is how long you can sell products (the longer is the better).