Temperature inside a unit can vary by as much as 10 °C from one place to another: the corners, the area in front of the cooling fan and the door zone all behave differently. Mapping therefore starts from a layout that catches all of these zones, not from a sensor count.
In the WHO guide the base is 12 fixed sensors: floor, mid-height and ceiling in each of the four corners. Up to 28 more go to risky points where product sits. A medium-sized cold unit generally gets 20 sensors plus one for ambient temperature, recorded for at least 48 hours.
A small shipping container is far smaller than a cold room. The same corner layout is taken as the frame, the extras go to the door, the cooling source and where the product actually sits, and the reason for every point is written into the protocol.
