Not so many (5)
I believe the answer is:
fewer
'not so many' is the definition.
('few' can be the opposite of 'many' so perhaps 'fewer' is reasonable for 'not so many')
This is all the clue.
(Other definitions for fewer that I've seen before include "smaller number of people" , "Less in number" , "in smaller numbers" , "Reduced" , "Not so many" .)