In Expressions^x, Countable nouns^x, Numbers^x

Hi, 1. I have two sentences below with the words (I they are variable nouns) and want to know why they use the genenal uncountable form when the alternative individually considered countable noun form seems to prevail just as well. I am asking this because when I see similar sentences I am...

Expressions such as: - a number of - a large/small number of - the number of - large/small numbers of are followed by a noun in the plural ("a number of" can refer only to countable nouns). "Large numbers of/A large number of teachers attended the seminar." "This killing brings the...