The bundles will tell you how much linear feet you get with each one. So if you need insulation to fit a 16" inch wide bay that is 12 inches tall in a 2x4 wall thickness....that is what you need to measure for.
So if your wall is 1 foot tall....how many pieces will you need at 16 inches wide? Count the bays.
That will be your linear feet.
Hope this makes sense, if not, feel free to ask again.