Covered porticoes line the Har HaBayit on all four sides. “The Har HaBayit was a double stav… a stav within a stav” (Pesachim 13b), which Rashi explains: one colonnade runs all around, and inside that circuit another. The model therefore sets two rows of columns along each wall. The same page speaks of the “roof of the itztaba”, on which the two loaves were placed; Rashi specifies that this roof was built against the rain, and the Rambam says the whole Har HaBayit was covered (Beit HaBechirah 5:1). The number of columns, their spacing, profile and height are choices.
Sources
Project decisionTour's choice: no column stands in front of the five gates of the Har HaBayit or on the east-west axis, to keep clear the view that the low east wall is there to allow (Middot 2:4).