https://www.theverge.com/2018/9/13/17855976/iphone-xs-max-xr-esim-technology-dual-sim-limited-support

The new iPhone XS and XS Max will have dual SIM capabilities, with a special edition for the Chinese market that contains a SIM tray which holds two physical cards. For users outside China, however, the new iPhone devices will support eSIM. While there are many benefits to eSIM, the nascent technology hasn’t seen wide adoption yet.

Only nine countries offer eSIM support right now: Austria, Canada, Croatia, the Czech Republic, Germany, Hungary, India, Spain, the UK, and the US. Sprint and smaller carriers in the US don’t offer eSIM support, meaning that customers will need to switch over to T-Mobile, Verizon, or AT&T in order to enjoy that feature.