I agree with B Spot here. Both factors, hair count and grafts/cm2 play an important role.
More hair equals more density - that's obvious. However, sometimes there is reliance on "hair layering" to give coverage. In other words, it lays down upon itself to prevent seeing through to the scalp - kind of a shingling effect.
Sounds good in theory, but in reality there is always some part of the scalp that is visible. It might be where you part your hair or when the wind blows or when wet or even at the edge of the hairline. When looking at those parts of the scalp that are showing, grafts that are too far apart look bad. It looks much better to look down on 40-50 or more grafts/cm2 sprouting out of the scalp than 20-30.
At the same time you also need an adequate hair count per graft, otherwise your entire hair will tend to be "see through".
I think Dr. Feller discussed this once a long time ago.
Gorp