How many are on number twenty? (9)
I believe the answer is:
fourscore
'twenty?' is the definition.
(thesaurus)
'how many are on number' is the wordplay.
'how many are' becomes 'four' (I can't justify this - if you can you should believe this answer much more).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'number' becomes 'score' (score is a kind of number).
'four'+'score'='FOURSCORE'
Can you help me to learn more?
(Other definitions for fourscore that I've seen before include "number" , "Eighty" , "''... and seven years ago, our fathers.... (Gettysburg address)" .)