How many 18 inch square tiles are needed to cover a floor that measure 12 square feet?

Answer 1

12 sq ft = 1728 sq inches

each tile is 18 x 18 = 324 sq in

1728/324 = 5.33 tiles

I assume you did mean 12 sq ft not a 12 ft by 12 foot floor which is 144 sq ft or 64 tiles

Answer 2

The answer depends on the shape that you are trying to cover since that will determine how many tiles will need to be cut. That will determine the wastage rate. But you will need a minimum of 6 tiles.