Please disregard the other answer given here - it is incorrect. This coming from a 23 year IT veteran who just recommended this model laptop to a family member.
The AMD Ryzen 5 3500U CPU with Vega 8 graphics supports 3 simultaneous monitors in total: The built in display, an external monitor via the hdmi port, and an additional external display through a usb type c 3.1 to display port adapter or cable.
According to the HP specification sheet, this model includes HDMI and a usb c 3.1 port that supports the display port 1.4 protocol. You will need the appropriate adapter from usb c to display port. I have included the lowest cost option available at bestbuy.com below, however these can be purchased from elsewhere as well.
From : https://press.ext.hp.com/content/dam/hpi/press/press-kits/2019/hp-reinvent-2019/datasheets/HP-ENVY-x360-Convertible-15m-ds0011dx-Datasheet-AMD.pdf
External notebook ports:
•1 USB 3.1 Gen 1 Type-C™ (5 Gb/s signaling rate, Power Delivery 3.0, DisplayPort™ 1.4, HP Sleep and Charge)(42)(50)
•1 USB 3.1 Gen 1 Type-A (HP Sleep and Charge)
•1 USB 3.1 Gen 1 Type-A (Data Transfer Only)
•1 AC smart pin
•1 Headphone/microphone combo
The displayport 1.4 specification will support 4k @ 120hz displays and 8K @ 60Hz, whereas the HDMI probably will support 4K @ 60Hz based on the AMD specifications for the Vega 8 on the 3500u