This is either a bug or flawed documentation scripting example.
From documentation it is unclear whether you have to activate the monitors first or not, as activating monitors first produces correct results, however
according to documentations' example, activating is executed after checking available display's count

To reproduce:
1. Open any project
2. Include the attached "DisplayScript.cs" (or create one using documentation script example)
3. Enter Play mode

Expected: Display.displays.Length returns available monitor count
Actual: Display.displays.Length always returns 1 (if not activating display beforehand)

Reproduced in: 2017.4.18f1, 2018.2.20f1, 2018.3.1f1, 2019.1.0a13

Note: related documentation page:
https://docs.unity3d.com/Manual/MultiDisplay.html