Actually there can be infinite number of rectangles. Lemme explain,
perimeter = 2(L +B) = 18
⇒L +B = 9
now, we can take any positive number, can be fraction also.
It should be noted that L or B cannot be Zero, because if it is zero, then rectangle could not be formed.
If it is asked, the length and breadth can only be a natural number, then possible dimensions are (length,breadth) as (8,1), (7,2),(6,3),(5,4)
then only 4 rectangles are possible
Otherwise, infinite number of rectangles are possible.