China has second-most billionaires, with Hong Kong, Beijing, Shenzhen and Hangzhou ranking among top 15 billionaire cities, according to research company Wealth-X's latest Billionaire Census report on Thursday.

China had 285 billionaires with their total wealth reaching $996 billion in 2018, while the United States had 705 billionaires, the highest, with net worth of $3.013 trillion, said the report.

In 2018, the global billionaire population fell 5.4 percent from a year earlier to 2,604 and their worth declined by 7 percent to $8.6 trillion. The report attributed the decline to heightened market volatility, global trade tensions and a slowdown in economic growth.

The billionaires are increasingly congregating in a cluster of cities – a total of 773 billionaires are living in the top 15 billionaire cities, accounting for nearly 30 percent of the global billionaire population in 2018, said the report. New York is the world's leading residence for billionaires with the figure reaching 105.

While the billionaires only accounted for one percent of the global ultra-high-net-worth individuals with over $30 million in net worth, but controlled almost 28 percent of its combined wealth in 2018, said the report.

Banking and finance, industrial conglomerates, real estate, food and beverages, and manufacturing were the top five primary industries that created billionaires.

Among the 2,604 billionaires, fortunes of 55.8 percent were self-made, 11.3 percent inherited their wealth and 30.9 percent from a combination of inherited and self-made.

The billionaires' average age was 65.7 years in 2018, with 10.4 percent of them under 50, 52.7 percent between 50-70 years old and 36.9 percent over 70.

The billionaire population remains heavily male dominated, with just over one female for every nine men. The ratio is only slightly lower among the ultra-wealthy population, said the report.

Philanthropy is the leading hobby among billionaires, and other interests include sports, politics and art, the report said.