How Many 18 X 18 Inch Ceramic Tile Do I Need To Cover 85.8 Square Feet?

Each tile is 1.5 ft square, so it has an area of 2.25 ft^2. You will need 85.8/2.25 = 38.13 of them.

At least 39 tiles are needed for the desired coverage.

You may find that you will need more tiles in order to finish the edges nicely.