Usually you should add 10-15% for waste. If you are going around base cabinets or through a doorway, or if your room doesn't allow for full tiles from wall to wall you will be doing some tile cutting. Also, don't forget the grout lines that are approximately 1/4 inch each. I would suggest an extra 5-10 tiles to allow for waste and mistake cuts.
Since a 12 inch by 12 inch tiles is 1 square foot, being 1 foot on each side, you can assume 40 sqft would be 40 tiles. However, if the area is 40 feet by 40 feet... you will actually need to multiply it. You will need 1600 tiles to fill that space!