500! Five hundred sheets! Imagine! (5)
I believe the answer is:
dream
'imagine' is the definition.
(dreaming is a kind of imagining)
'500 five hundred sheets' is the wordplay.
'500' becomes 'd' ('d' can be a synonym of '500').
'five hundred sheets' becomes 'ream' (I've seen this before).
'd'+'ream'='DREAM'
(Other definitions for dream that I've seen before include "Cherished hope; fantasy" , "Cherished hope or ideal" , "King had one" , "Nocturnal vision" , "Something imaginary" .)