The Long Tail is the other half of the same chart we saw in an earlier Big Ideas article: the Pareto Principle. Vilfredo Pareto took an interest in the head of the chart, where a small number of people accounted for a huge proportion of wealth. In 2006, Chris Anderson published a book that looked at the huge number of products that form the remainder, when you exclude the few that account for the majority of sales.
How many categories of item does Amazon list? I found the statistic from January 2018 that:
Amazon lists more than 3 billion products across 11 marketplaces worldwide.
But most of their sales and revenue come from the top few highest performing goods in each category. A small number of books may sell thousands a month. But there are millions of books that rarely sell any in a given month… or year.