Number of sheets — about a thousand? (4)
I believe the answer is:
ream
'number of sheets' is the definition.
'ream' can be an answer for 'sheets' (I've seen this before). I'm not sure about the 'number of' bit.
'about a thousand?' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'thousand?' becomes 'M' (Roman numeral for a thousand).
're'+'a'+'m'='REAM'
Can you help me to learn more?
(Other definitions for ream that I've seen before include "480 or 500 sheets" , "516 sheets of paper" , "500 sheets, perhaps" , "Widen out (bore)" , "480 sheets of paper" .)