If the model is in the garrison, you measure to the garrison to determine it's range and visibility. So for each model that's in the garrison, you can check range to the objective from the garrison.
So if you have 10 arkanaughts inside an Ironclad that's sitting on an objective, each of those 10 arkanaughts would count as being within 6" of the objective because the garrison rule says to determine range for them by measuring to/from the garrison.
Nothing implies counting them as 1 model.