.
The words "around the outside" make me stupid, because it is not a math term.
I would understand "around the perimeter" . . . In my view, it is much better description.
If you accept this conception, then calculate the perimeter
P = 4*6 = 24 feet,
convert it to inches P = 24*12 inches,
divide by 6 to get N =
= 24*2 = 48 intervals between the plants.
Thus your answer is 48 plants, same as the number of intervals.
Answer. 48 plants.
Notice. If you do not accept my conception, do the same.