To supplement flebourse's comment, WebUI (at Network > Interfaces) doesn't show certain interfaces, such as WireGuard, OpenVPN, loopback and some others. However, as mentioned above, ifconfig in the CLI will show you all single virtual or physical interfaces.
I'd also recommend to check out the "iproute2" utility - specifically for this case run command "ip address" in the CLI. This will show you all interfaces, not just currently running ones, in addition to alias interfaces. You can also run some further flags to reduce the output, for example, command:
ip address show up
will show all "up" interfaces. Further modifications to the command could be made, by telling this utility to show only IPv4 protocol-related information:
ip -4 address show up