Countless millions to back milk producer (6)
I believe the answer is:
myriad
'countless' is the definition.
('myriad' can be a synonym of 'countless')
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for myriad that I've seen before include "Many" , "Large indefinite number" , "Host" , "Umpteen" , "a vast number, perhaps innumerable" .)