Adverbs in general can be difficult as they can appear in different places around the verb.
He ran quickly to the store. Or He quickly ran to the store. Both are techincally correct, but the first one sounds better to the ear.
Here, the tip off is right after "most" - it's a prepositional phrase "of them". "them" referring to the 80 massive planets. So in the sentence, "most" is an adjective quantifiying how many of "them".
Look at this one: "That answer is mostly correct." "Mostly" modifies "is"...To what extent is it correct? Mostly. At first glance, it appears mostly and correct go together, because it sounds like it modifies "correct". Most adverbs will answer the question "To what degree/extent does the action/verb take place?" My hypo of "That answer is mostly correct." might not be the best example.
I'm not sure how often this rule will be correct, but it's something to think about. If "most" is right after a verb, first make sure the verb modifies the subject and is not another adverb by itself, then ask yourself if "mostly" answers the question "To what degree or extent does this action take place?"
You'll also find this with infinitives, and really deals with adverbs that end in -ly. Everyone knows the phrase "To boldy go where no man has gone before." This is not gramatically correct. Boldy is an adverb, saying how they "go", but first, the infinitive should not be split, it should be "to go boldly", again, you find the adverb after the verb "go". I hope this helps and hasn't made it more confusing.
As for 'mostly' modifying '65,000 units'...i'm not sure the question "To what extent/degree..." really helps out in this situation. What does help out is that a prepositional phrase immediately follows "mostly". "FOR CARS" is a prepositional phrase. If you split up the concepts being presented into separate sentences, it might help figure out where the modification belongs.
The joint venture, Automotive Energy Supply Corp., expects production of 65,00 units annually by 2011. The 65,000 units are mostly for cars. Here, "mostly" modifies the verb "are". It's still the same concept as in the sentence, and shows you that it is not modifying "65,000 units" or "for cars".
It has to be modifying "expects".
I don't have all the correct labels for every part of speech, but this makes sense to me. If I've confused the hell of out of you, please tell me and I'll come back and delete this post

'The joint venture, Automotive Energy Supply Corp., expects production of 65,000 units annually by 2011,
mostly for cars.' -- Source: WSJ
Here, isnt 'mostly for cars' modifying '65,000 units'?? --> that, the 65,000 units are 'mostly for cars'
or is it modifying 'expects'??
gmat blows
Can someone shed some light on the use of adverbs, in particular, 'mostly'.
In the past several years, astronomers have detected more than 80 massive planets, most of them as large or larger than Jupiter, which circle other stars.
(A) most of them as large or larger than Jupiter, which circle
(B) most of them as large or larger than Jupiter and circling
(C) most of them at least as large as Jupiter, circling
(D) mostly at least as large as Jupiter, which circle
(E) mostly as large or larger than Jupiter, circling
Source: GMATPrep, OA: C
I have read that 'mostly' is incorrect here since it is an adverb and therefore should only modify verbs/adjectives/other adverbs. Since, this phrase is intended to modify the '80 massive planets', 'mostly' is incorrect
'The joint venture, Automotive Energy Supply Corp., expects production of 65,000 units annually by 2011,
mostly for cars.' -- Source: WSJ
Here, isnt 'mostly for cars' modifying '65,000 units'?? --> that, the 65,000 units are 'mostly for cars'
or is it modifying 'expects'??
Thanks!