https://techcrunch.com/2018/09/12/the-iphone-xs-and-xs-max-get-dual-sim-capability/

There are many reasons why dual-SIM capabilities make sense. And that’s why many Android smartphones let you insert two SIM cards. Apple is entering the world of dual-SIM capabilities with a physical SIM tray and an eSIM for most of the world, and two physical SIM cards in China.

You won’t be able to buy a second SIM card at the airport and put it in the phone. Instead, just like on the iPad, you’ll have to subscribe to a plan using your iPhone. Few telecom companies support eSIM just yet. Apple showed the logos of Verizon, T-Mobile, AT&T, Bell, EE, Vodafone, Airtel, Deutsche Telekom, Truphone, GigSky and Jio.