12 pieces are in the bag or 116.25 sq. tt.
The label says 116.25 sq ft.
Please wear safety glasses, long sleeve shirt and gloves when installing. If you are installing in an attic, you may want a hard hat due to nails in the rafters.
The higher the density, the higher the R Value. R15 has a higher density of glass than an R13. The same with R21 verses R19.
You'll want to peel off an inch of the insulation instead of compressing it. It will fit if compressed, but you will lose more R value than if you peel off a layer.
You should only use one vapor barrier. If you use sheeting you should remove the kraft paper from the insulation. Else, moisture can collect between the vapor barriers.
If you have 2 x 4 walls it is what you need provided your basement is reasonably dry. If you have a wet basement you would probably be better using polystyrene.
I would recommend using ridged polystyrene because of the potential of humidity causing mold. If you use fiberglass you would have to use a poly vaper barrier against the block.
116.25 sq. ft. per bag 12 pieces of 15"X93" batts in each bag. Square footage is located in product overview on Homedepot.com.
Depth 50", Height 17", Width 16" weight of the bag is 29.3 pounds