In order to estimate the number of banknotes in active use in the world, we would need some key data points:

1. The number of banknotes in circulation for each currency: This information would be available from the central banks of each country. For example, the Federal Reserve in the U.S. publishes this information for the US dollar.

2. The proportion of banknotes that are actually "in use": Not all banknotes in circulation are actively used. Some are kept as reserves or as collectibles, while others might be lost or damaged. The proportion of banknotes in active use can vary greatly depending on the currency and the country.

Without these data points, we can't calculate the exact number of banknotes in active use. However, I can show you how to make the calculation assuming you have the necessary data.

Let's assume you have data for 3 countries (for simplicity) - the USA, UK, and Japan - in billions of banknotes. The numbers are hypothetical.

- USA: 50 billion banknotes 
- UK: 30 billion banknotes 
- Japan: 20 billion banknotes 

You also have data saying that, on average, 70% of these banknotes are in active use. 

So, the calculation would be:

- USA: 50 billion * 0.7 = 35 billion 
- UK: 30 billion * 0.7 = 21 billion 
- Japan: 20 billion * 0.7 = 14 billion 

Adding these up, we get 70 billion banknotes in active use in these three countries. 

If you want to calculate in millions, you can convert it by knowing that 1 billion is 1000 million. So, 70 billion banknotes would be 70,000 million banknotes. 

This is a simplified example, and the real calculation would involve data from all the countries in the world, each with their own proportion of banknotes in active use. Also, the number of banknotes in circulation is always changing due to factors like inflation, changes in demand, and the introduction or withdrawal of certain denominations.
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