I have seen many property investing Ads that claim that one can buy and own more than 30 properties with little or no money down. In your view, is this possible? If yes, how can one do so, and what are the pitfalls of doing so?

Very difficult.

those adverts are saying "no money OF YOUR OWN down".

they are not saying the property can be bought without money down, its teaching you how to use other people's money via loans, partnerships etc.

not very ideal when property prices stop rising and banks start being more stringent on payment.

If it sounds too good to be true, it's probably too good to be true. You can also own more than 30 companies with little money down, by buying 1 share in 30 different U.S companies by maxing out your credit card. I advise most young people to focus on what's time-tested.

If you really really feel tempted, i would say go attend the free seminars that they typically offer, just to look-see. And check out MAS IAL list (https://www.mas.gov.sg/investor-alert-list)