The royal made the comments during a visit to the Gardens by the Bay attraction in Singapore, where the Duke and Duchess of Cambridge are making an official visit.
Corine Ackermann, 17, who was there with friends from the Tanglin Trust British School, told The Sun: "Someone asked him how many children he would like to have, and he said he was thinking about having two."
The Duchess is said to have sparked rumours she is pregnant after drinking water from a wine glass and later "holding her stomach in a maternal way".
The couple are currently on a nine-day tour of the Far East and South Pacific in honour of the Queen's Diamond Jubilee.
Kate was recently said to be supporting Prince Harry through his naked photo scandal.