1 per box
Sash Weight of 17 to 20 LB. per cable
HOW DO I DETERMINE THE WEIGHT?
Remove the sash from the window frame, and place it on bathroom scales. If it weighs 13 pounds, you want a counter balance of a little more than that weight.
Because two cables for two balances are needed, each side should be half of the weight, so buy two 8LB balances. You will have lift of 8 pounds per side, or 16 pounds, enough for the 13 pound sash.
Note the Sash Weight range is below each item. ONE box contains TWO weights, enough for a single sash window.