As far as I know, all the words can be used as a pronoun. 'many' and 'few' occur with countable nouns, while 'much' and 'little' with uncountable nouns. There are two confusing sentences, which I saw in a grammar book.
(1) I'm not very busy today. I haven't got much to do.
(2) This is a very boring place to live. There's little to do.
Why both sentences don't use 'many' and 'few' instead? I think 'many' would represent 'many (things)' in the first sentence , and 'few' would represent 'few (things)' in the second sentence. Is my understanding right? Or actually there are some collocations which need 'much' and 'little' with the to-infinitive.