It says so on the base (5V, 2.5A so it can handle 12W).
However this doesn’t rule out it supporting more (Apple could have downplayed its specs) BUT: fast charging above 12W requires a dual-sided Lightning port. If you look inside the port on the dock it’s only single-sided, so the limit must be 12W.
EDIT: Actually it looks like the iPhone X has a single-sided Lightning port and it supports 18W, so it’s possible (though unlikely) the dock actually supports 18W.
It says so on the base (5V, 2.5A so it can handle 12W).
However this doesn’t rule out it supporting more (Apple could have downplayed its specs) BUT: fast charging above 12W requires a dual-sided Lightning port. If you look inside the port on the dock it’s only single-sided, so the limit must be 12W.