The amount of money is not fixed. The amount changes every single day thanks to interest payments and the fractional reserve system.
Banks in the U.S. are only required to have about 10% of their deposits on hand at any particular time. The other 90% is loaned out, which becomes "money" for someone else (seller of a house, car dealer, etc). Then, that bank loans out 90% of that money, and the cycle continues...
Therefore, banks create money all the time. Most other economies are based on this fractional reserve system as well.