Good question Bramp! Most small sail boats have only one sail because the square footage of sail area is big enough to collect enough wind to push a light boat. As the size of the boat gets larger and heavier, you need to have more sail surface for the wind to interact with - the sail has to be larger. At some point, the sail gets just too large to handle. The solution is to add more sails that are smaller and easier to handle but collectively make up a large sail area.

The second part of your question has to do with a phenomenon called apparent wind. Apparent wind is the wind you feel on your face as you move forward. True wind is the wind that is blowing naturally.

If you can imagine riding your bicycle on a day when there is no wind whatsoever, you still feel wind on your face (apparent wind) and it gets stronger as you go faster. That is because your forward motion is creating its own wind. If you were to ride your bike on a day when there was a 5 mile per hour wind behind you and you were pedaling at 5 miles per hour, the two winds (true and apparent) would cancel each other and you would not feel any wind at all.

Boats that are able to sail faster than the true wind are "creating their own wind". Generally these are fast catamarans and iceboats, although some racing monohulls may be able to achieve this. The apparent wind is the wind that the boat sails in. Usually, you can sail faster at 70° to 80°off the apparent wind (called a "close reach") than you can with the wind directly behind you.

This is because you can trim the sails so that the wind flows over them to create a lift, much like an airplane wing, that propels the boat. As you can see, there is a positive force against the inside of the sail, and a negative force pulling the outside of the sail. (You can try this by holding your hand out of the window of a moving car (With your parent's permission, please!). Rotate your hand to feel how the wind pushes and pulls on it at different angles.)

Under optimum conditions, the apparent wind is greater than the true wind. Let's say you are on a fast catamaran and sailing in a true wind of 10 knots. By moving very fast through the water you may be able to create an apparent wind of 20 knots which may allow you to sail at 12 to 13 knots, which is faster than the true wind. (Friction will keep you from moving as fast as the apparent wind.)

This is a very simple explanation; many books have been written on this subject and if you have any interest in racing you will want to learn more about this than can be explaned here. Also, see How Sailboats Sail.