Havocscope, a provider of data on the black market, in a 2015 book titled Prostitution: Prices and Statistics of the Global Sex Trade, wrote:

“Over $180 billion is spent each year on the global sex trade, with over 10 million women providing services as prostitutes. Some are forced into the trade due to human traffickers, while others enter the trade due to financial hardships…

The number of prostitutes by country were collected from a wide range of public sources. Among the sources included security services estimates, reporting by public health programs, and other monitoring data from global criminal justice programs.

China 5,000,000
India 3,000,000
United States 1,000,000
Philippines 800,000
Mexico 500,000
Germany 400,000
Brazil 250,000 children
Thailand 250,000
Bangladesh 200,000
South Korea 147,000
Turkey 118,000
Taiwan 100,000
Cambodia 70,000
Ukraine 67,500
United Kingdom 58,000
Kenya 50,000 children
Vietnam 33,000
South Africa 30,000 children
United Arab Emirates 30,000
France 20,000
Switzerland 20,000
Poland 19,000
Mongolia 19,000
Israel 17,500
Costa Rica 15,000
Netherlands 7,000
New Zealand 3,500
Denmark 3,200
Ireland 1,000.”