Question 549003
well depends on the diameter of the plant because the numbers will change if the plant is 12 inches in diameter or 16 inches in diameter... or do u mean 12 inches from the center of each plant?

the area is {{{36 * 72= 2592in^2}}}
72-24=48 the 24 inches are the space from each side leavein 48 inches
36-24=12
so if u visually draw a rectangle and we are going simply by measuring plants from center to center to side then there are only 2 rows high 
48/4=12
my crap drawing below is indicating that theres 12 inches from the sides to the center of the plants then theres 12 inches between each plant and each dot(*) represents a plant center. 
if we go by the plant centers then there can be as many as 10 plants in this rectangle
but if we are to be acurate by each plants diameter then there will be much less plants that will fit and that diameter information will be needed.

 _________________
|--12--12--12--12--12--|
|12*---*---*---*---*12|
|-12--12--12--12--12--|
|12*---*---*---*---*12|
|_12--12--12--12--12_|

above shows 12 as inches between dots its 3 feet high and 6 feet wide
i tried to draw it on here, much easier if i could upload a drawing.