Numbers gathered or classed together (6)
I believe the answer is:
groups
'numbers gathered or classed together' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for groups that I've seen before include "Numbers gathered or classed together" , "Numbers of associated individuals" , "Numbers of people or things classed together" , "Numbers of entities considered as units" , "Lots of" .)