You know the rule, nearly all revenue (80%) comes from a fifth (20%) of your customers. That is also true for Federal and State tax revenues. It is only a rule of thumb .... but almost a law of business.
David Boxenhorn pointed out that the rule applies to two more levels of arithmetic: roughly 60% of business comes from 5% of customers, and 50% comes from 1% of customers.
This latter figure is interesting, because in the old music business, book publishing, the current film business and in venture capital, it is just one investment, one work of art that comprises nearly the entire business.