Jessica,

If 32x48 is the size of the sheets (presumably in inches) then (a) you do not need to know this information, except to round up to the nearest whole number of sheets; and (b) you do need to know the gauge and composition of the sheet metal, which you have not stated.

Once you know the thickness, it is fairly easy to find tables that give lbs per square foot for various thicknesses and types. There is one here:

http://www.auto-ware.com/techref/sheetweight.htm

and books like "Pocket Ref" probably have them too (my copy's at home.)
Once you have the figure, divide 25,000 by it to get square feet.

Each of your sheets is 32 × 48 = 1536 square inches which is 1536/144 = 10 ²/₃ square feet. If you have to purchase whole sheets, divide the number of square feet by 10 ²/₃ to get the fractional number of sheets and round up to the next whole number - that's the number of sheets you'll actually need.

-RD