18 inch x 18 inch tile is really (1 and 1/2) feet by (1 and 1/2) feet
That is (3/2) x(3/2) square feet = 9/4 square feet.
To cover 900 square feet we need (900)/(9/4) = (900) x (4/9) = 100 x 4 = 400 tiles
10
67 tiles.
It is not clear what is meant by an "18x18 square feet" tile. 18ft * 18 ft = 324 square feet, so that one tile will be more than enough. An 18 inch by 18 inch tile cannot be a 18x18 square feet tile.
288
18x18 what? 18 feet x 18 feet? 18 microns x 18 microns?
10
67 tiles.
start with 38 tiles.
192 tiles
288
118
It is not clear what is meant by an "18x18 square feet" tile. 18ft * 18 ft = 324 square feet, so that one tile will be more than enough. An 18 inch by 18 inch tile cannot be a 18x18 square feet tile.
12...
18x18 what? 18 feet x 18 feet? 18 microns x 18 microns?
30 tiles! But you'll need more than 30 tiles to cover wastage, breakage, etc.
To cover 720 sq feet of floor with 18" x 18" tile, you would need 320 tiles.
The answer will depend on the units used for measuring the tiles which you have failed to specify.