How much material (54" width) do I need to make (8) 16"x16" cushions?

Assuming the cushion covers will consist of two square pieces of cloth stitched together, without a separate edge/side piece...

You will need 16 squares (8 squares, a top and bottom for each), every square approximately 17.5" on all four sides, cut straight with the grain of the fabric. This gives you a 3/4" seam allowance, which should be plenty, large enough to add a zipper if you want to be able to remove and wash the covers.

You can fit 3 squares across 54"-wide fabric, with a small amount of waste at one edge. 16 divided by 3 equals 5 with a remainder of 1--five rows of squares, three squares per row, plus one more square, for a total of six rows each 17.5" long. That's 105", or 2.91666 yards of 54" fabric. I'd buy three yards.