The revelation of the future king's prospects for fatherhood came during a chat with bystanders, in which he was asked how many children he would like to have.
"He said he was thinking about having two," said Corine Ackermann, 17, who was with school friends as the duke and duchess of Cambridge toured the Gardens by the Bay, the island state's newly opened tourist attraction.
Prince William has said previously he wanted to start a family, but he had never detailed how many heirs he desired.
About 3,000 people, including some 50 children from the Tanglin Trust British School, came out to the attraction to cheer, take pictures and wave Union Jack flags.
Jaz Heber Percy, 13, asked the duke what power he would like to have if he could be a superhero.
"That's a hard question," the duke said. "I'm not sure. I'll have to think about it. I think invisibility."
When the same question was posed to his wife, she said that she too would pick invisibility so that the prince "could not sneak up on her."
After leaving the gardens, the pair went on to unveil the first airplane engine to be produced by Rolls-Royce Plc at its new factory in Singapore. The duchess pushed in the last fan blade as a symbolic inauguration of the production of the Trent 900 jet engine that powers the Airbus A380.
In his speech to the gathering of Rolls-Royce executives, workers and guests, William highlighted the historic relations between Britain and the former colony of Singapore.
"It is no accident that Rolls-Royce has chosen to build this high- tech campus in Singapore. This country's welcoming business environment, highly skilled workforce and entrepreneurial energy have made it a magnet for many hundreds of British companies," he said.
The royal couple visited the Queenstown residential area, named after William's grandmother, Queen Elizabeth II, at her coronation in 1953.
Residents and onlookers eagerly awaited the royal couple despite the tropical heat, cheering and waving British flags as the pair arrived.
They were received by traditional lion dancers and treated to other performances from the ethnic Indian and Malay communities before they went on to visit The Rainbow Centre, a school for children with multiple disabilities.
In the evening, the couple was to be guests of honor at a dinner for the British business community hosted by the British high commissioner.
They were scheduled to visit the Kranji War Memorial early Thursday before flying to the Malaysian capital Kuala Lumpur. The royal couple will then go to the Solomon Islands and Tuvalu.
The Singapore visit is first stop on a nine-day tour to four Commonwealth countries in the Asia-Pacific region as part of the celebrations of the queen's diamond jubilee, marking her 60 years as monarch.
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