How can there be so many different substances in the world if there are only a few elements that are common?

-- If there are only  10  elements in the universe that can make compound molecules, and a compound molecule can be formed by combining  1,  2,  3, or  4  different elements, then that's already the possibility of  at least 400 different molecules.

-- There are many more than  10  elements that can combine to form compound molecules.

-- Every single "organic" molecule, of which there are thousands, is the combination of carbon with other elements.

-- Most all of the substances that can be distilled out of oil, including the paraffin waxes, the alcohols, gasoline, kerosene, butane, propane, octane, and natural gas, are made of just carbon, hydrogen, and oxygen, only with different numbers of each one. 

-- Plastics, drugs, rubber, and DNA are examples of molecules that are made of hundreds of atoms.