it is not the amount of plants that matter, its the size of the room you are exhausting .
H x W x L = Volume
this will give you your Cubic feet of your room. then you add 20% and match filter CFM to a fan with the same CFM .
for instance
4 x 4 x 8 = 128CuFt
128 + 20% = 160 CFM fan and filter combo.
J