I have some old recipes that call for a box of powdered sugar. How many cups would that equal ? I buy powdered sugar in two pound bags.
My best guess would be one pound, since it's still often sold in stores in 1lb boxes, regardless of brand:
You can see plenty more like that if you do a Google image search for [powdered sugar box].
As others have pointed out, measuring by volume is inaccurate (one pound could be 3.5 to 4.5 cups), so you'd be best off using a scale, or perhaps measuring your two pound bag into two equal volumes.
That said, if your recipe can tolerate being off by a couple ounces (around half a cup), you could use four cups for a one-pound box.
According to King Arthur Flour's Master Weight Chart, 1 pound (the most likely size of a box in the US) of unsifted powdered sugar is about 4 cups. As I said in a comment above, I highly recommend weighing powdered sugar instead of measuring it by the cup.
Per Domino® Sugar,
The answer to how many cups are in one pound of powdered sugar will vary depending on if your powdered sugar is sifted or unsifted. In general:
1 pound = 3 1/2 to 4 cups unsifted powdered sugar
1 pound = 4 1/2 cups sifted powered sugar
Customarily sold in boxes in the US, as in the illustration provided by @Jefromi, they are one pound/16 ounces. For a 2-pound bag, either measure out 3 1/2 to 4 cups for the recipe or, more straightforward, half of the quantity in the bag.