Hi Christian
The watts refer to the power supply unit that is supplied (the notebook is charged while it is connected to the docking station). 90 watts is easily enough for your T440s. The possible number of screens depends on the graphics installed in the book. I run the same DS with a T540 and two 24-inch Samsung displays (1920x1080) via Displayport: runs flawlessly at 2x 60hz. So that should be fine for you.

Greetings
Chris

The docking station is the same from my point of view, the required power depends on the design of the laptop (processor, graphics card). For the number of screens, the power should not play a role.