How to Real Estate Investors acquire thousands of properties overtime?

[Vaishal Patel]

I have read about people who started real estate investing with very little money and now own hundreds of properties.Last week i was reading about a landlord who owns over 1500 rental apartments,20 years ago he was almost bankrupt and now he owns 1500 rental apartments..what strategy do they use to acquire such huge number of properties?I know they borrow alot of money at the beginning but still what exact strategy/ways these guys do this?Thanks :)