How many lamps are in the 7-meter long hallway?

You can definitely write a doctoral thesis about that, but honestly, you can also do without it. We only have one ceiling outlet and an expensive double spotlight - we just never turn it on, because we have three wall spots on the stairs (bottom, middle, top) that we use instead. On at the bottom and off at the top is called a two-way switch and should also be included in the BT/GU for stairs. I would divide three over 7 meters.

Just because you deal with the basics of light and lighting doesn't mean it's a doctoral thesis. Personally, I just did a bit of research back then due to the sometimes really confusing information situation in order to get a better assessment for myself, and that should be feasible for almost anyone who otherwise asks questions like in this thread. You can of course just install something, but then it can also be completely wrong. It gets even more interesting when we talk about Smart Home and the lights are to be turned on and off automatically, but that was not asked here.