This scenario seems unlikely. As written, an 18 x 18 foot tile would cover 324 square feet and you'd only need a little more than one to do the job. Even an 18 sq. ft. tile would be unwieldy, although you'd only need 25 of them to cover the space. I'll assume you mean 18 x 18 inches, which is 2.25 square feet. You'll need 195 of those, but get 215 to be safe.
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It depends on whether the tiles are 18 cm * 18 cm, or 18 in * 18 in or any other units of linear measurement which the question does not specify.
157 18 by 18 tiles are needed to cover 352 square feet.
The answer will depend on the size of the tiles. Since there are no units given, the fact that the tiles are 18*18 is useless for answering the question.
You will need at least 67 tiles. This will cover 150.75 square feet.
To cover 720 sq feet of floor with 18" x 18" tile, you would need 320 tiles.