The test is wrong.
Clocks are countable nouns (things that can be individually counted). Countable nouns can be quantified with few or many.
Either of these sentences would be grammatically complete:
There are many clocks in the office but few of them work properly.
There are many clocks in the office but many of them work properly.
The second sentence sounds a little awkward, due to the repetition of many, but it is grammatically fine. Maybe you asked your clock repairman if it would take a long time to repair all the clocks in the building. He responds "Not long - There are many clocks, but many of them are working fine [and don't need repair.]"
Suppose instead you were discussing sand on the floor. Sand is not generally thought of as countable, and you would use little or much.
There is a lot of sand in my goldpan but little of it is likely to contain any gold.
The winds weren't too strong. There is a lot of sand on the office but much of it was tracked in by the dog after the storm was over.