1. Most roofs aren't flat.
2. For pitched roofs, you're only going to want to install on the south or west facing sides. So only some percentage of your roof area is ideal.
A pitched roof and installing on only half doesn't change it that much. If your roof is 45deg then you're talking about a 24x24 building. If that's two stories this is 1152sqft, still less than half the size of the median US house.
There are roof designs where even on a large house 21 panels won't fit, but looking around my neigborhood most houses could easily do that many.
There are roof designs where even on a large house 21 panels won't fit, but looking around my neigborhood most houses could easily do that many.