a + adjective + "many" + of the + plural noun

(Countable noun) [a + adjective + ~ + of the + plural noun] a large number of persons
or things:

A good many of the beggars were blind.

https://www.wordreference.com/definition/many

Is the plural nominal complement compulsory, so that A good many were blind is not grammatically correct?

Secondly, is it correct to label it as a "countable noun"?