The correct quantifier in this context is "many".
The word "many" is used with countable plural nouns such as "millions". Since "millions" is countable, "many" is the appropriate choice.
The sentence becomes:
Brazil earns many millions of pounds a year from coffee exports.
The correct sentence is "Brazil earns many millions of pounds a year from coffee exports."
Among scientists, the discovery of the double helix structure of DNA and the genetic code it incorporates is widely regarded to be one of the most significant scientific discovery of the twentieth century.