Vision of 500 — that many sheets? (5)
I believe the answer is:
dream
'vision' is the definition.
(dream is a kind of vision)
'500 that many sheets?' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'that many sheets?' becomes 'ream' (many sheets of paper).
'd'+'ream'='DREAM'
'of' acts as a link.
(Other definitions for dream that I've seen before include "Nocturnal vision" , "Armed (anag)" , "Think vaguely" , "Distant ambition" , "Sleeper's vision" .)