Monetary Unions

Wikipedia

The Euro is often touted as something unique in history, but its not the monetary union that makes it unique. Since 1945, two sizable monetary unions have existed in Africa. Even in Europe, such unions have not been unique. In 1865, the Latin Monetary Union was established. Since, at that time, most countries used gold and silver coinage, the union simply standardized the amount of gold and silver that could be used to make coins to make them interchangeable. This union included all of southern Europe and Venezuela. Even the US used the same standard, but never joined the union. The union eventually collapsed because the rising value of metals and WWI.

Following is a ranking of the all current monetary unions (official and de facto) in order of how many countries are involved. I am not including territories on this list.
Euro - 18
CFA Franc BCEAO - 8
US dollar - 7
CFA Franc BEAC - 6
East Caribbean dollar - 6
South African rand - 4
Australian dollar - 4
New Zealand dollar - 3
Swiss franc - 2

This means that among the 194 widely recognized countries, there are only 145 currencies. With the continued expansion of the Euro and other monetary unions, the number of currencies will drop to 119 in the near future.