In fact it has enough bandwidth to handle 4 1080p displays, but that's not officially listed as supported, and even if it works it might require additional hardware (such as a DisplayPort MST hub connected to the dock) and of course a system whose GPU supports 4 displays. Other than that, you can get 3 displays either by using the laptop's built-in panel with your 2 external displays, or if your 5480 has Thunderbolt 3 (optional on that model), you could get the TB16 dock, which will allow 3 external displays up to 1080p or 2 displays up to 4K because Thunderbolt allows 4x as much display bandwidth as a USB-C connection that is carrying display and USB traffic simultaneously. That is indicated on the WD15's product page, the manual available on site, and the WD15's FAQ page here: If you want 3 displays, the last page of the manual does list that as a possibility, but only at pretty low resolutions. The laptop and its GPU can indeed handle 3 displays, but due to bandwidth limits of regular USB-C when it's being used for both display and USB traffic simultaneously (as is the case with a dock), the WD15 only receives enough bandwidth to support 2 displays up to 1080p.
