start with 38 tiles.
The answer will depend on the units used for measuring the tiles which you have failed to specify.
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.
The answer depends on the measurement units used for 18x18.
22.2 tiles so buy 23 tiles.
100 tiles or 96 which is the correct amount