The word passus means two things that are best kept separate:
A step or a pair of steps.
A unit of distance, five Roman feet.
Meaning 2 is inspired by meaning 1 but not tied to it.
As a unit of distance it does not mean a pair of steps, but about 1.5 meters.
Different people and different situations will give you vastly different lengths of step, but the meaning of passus as a unit will always mean the same.
This is similar to how "foot" (either Roman or modern) means a specific length, not really the length of a foot.
The L&S entry on passus quotes Pliny (Plin. 2, 23, 21, § 85) saying stadium centum viginti quinque nostros [e]fficit passus, hoc est pedes sexcentos viginti quinque, so 625 pedes is 125 passus.
The exact meaning of 1 (whether in the context of walking passus means one or two steps) is irrelevant for meaning 2.
Of course two steps sounds more reasonable than one for that length, but requiring exactly two human steps is not what defines the unit.
Because passus as a unit is about 1.5 meters, mille passus is about 1.5 kilometers.
This is very similar to the modern mile but not exactly the same; the figures I have seen for the Roman foot vary a bit but not enough to make the two units agree.
Depending on who's walking and how, the number of steps required might range between one and ten thousand.
If you want to figure out whether passus means one or two steps, we probably need a passage from a classical author comparing passus other step-related words like gradus, gressus, incessus.
If that is your main focus, perhaps another answer can handle it.
This one's main point is that the meanings 1 and 2 above are distinct.
