It is an idiomatic expression: https://dictionary.cambridge.org/it/dizionario/inglese/good-many
– user 66974Aug 07 '22 at 07:33
1
Have you looked in any dictionaries? Many include the phrase "a good many". (If you are looking for its etymology, then please clarify.) Also, that phrase is usually construed as plural.
– MarcInManhattanAug 07 '22 at 08:35
I am not convinced that the singular usage is common. Can you quote sources?
– AntonAug 07 '22 at 10:49
1
It's bad grammar. Not the "a", but "is". It should be "There are a good many reasons why people should follow it."
– Hot LicksAug 07 '22 at 12:06
@Anton *A* few possible *ʀᴇᴀꜱᴏɴꜱ ᴅᴏ* exist for using the nominally singular indefinite article a/an with quantified plural nouns without this having any effect at all on their inherently plural grammatical number for verb agreement purposes, and some of these are explored in answers to the selected duplicates that *ᴀ* lot of *ᴘᴇᴏᴘʟᴇ ᴀʀᴇ* investigating even now as we speak. :)
– tchristAug 07 '22 at 14:36