"It" is the black market, or System D, a slang phrase adapted from the French word débrouillard. A débrouillard is a resourceful and self-reliant person. A débrouillard figures out how to get what they need regardless of the obstacles. The obstacles are usually the laws or price controls put in place by the state.
There are a lot of débrouillards in the world. In 2009, the Organization for Economic Co-operation and Development (OECD), estimated that around 1.8 billion people – at the time, half the world's working age people — had unofficial jobs that weren't registered, regulated, or (in many cases) taxed. The OECD estimated that by 2020, two-thirds of the world's workforce would be part of System D.
The OECD considers anyone between the ages of 15 and 64 to be "working age." As of mid-2018, about 65% of the world's 7.7 billion people were working age; that's about 5 billion people. If half of them rely on System D to support themselves and their families, that comes to 2.5 billion people.