Number of sheets — about a million? (4)
I believe the answer is:
ream
'number of sheets' is the definition.
'ream' can be an answer for 'sheets' (I've seen this in other clues). I'm unsure of the 'number of' bit.
'about a million?' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'million?' becomes 'm' (abbreviation).
're'+'a'+'m'='REAM'
Can you help me to learn more?
(Other definitions for ream that I've seen before include "500 sheets, perhaps" , "Lots of paper" , "Widen out (bore)" , "480 or 500 sheets" , "Many sheets - enlarge hole" .)