If it's really square you should need 156 18" tiles to cover 350 square feet. Most tile stores let you return unused tiles, so I suggest you get a few extra. The number needed can change based on the grout space, how many you break and if the area isn't square you may need more. A rule of thumb is to add 10% for waste. That rule would say to buy about 175.
start with 38 tiles.
118
The answer depends on the measurement units used for 18x18.
22.2 tiles so buy 23 tiles.
18 inches = 1.5 ft, so each tile is 2.25 sq ft, number required = 249/2.25 = 111
67 tiles.
192 tiles
start with 38 tiles.
10
288
118
12...
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.
220
The answer depends on the measurement units used for 18x18.
18" x 18" = 1.5' x 1.5' = 2.25 square feet90/2.25 = 40 tiles.