that has one form but five functions:
[1] demonstrative pronoun; e.g.,
That book is expensive.
[2] substantive noun; e.g.,
That is expensive.
[3] conjunction; e.g., He said (
that) he was fine.
[4] relative pronoun; e.g., books
that have been sold
[5] adverb, synonymous with 'so', meaning, to such a degree; e.g., The fish you caught wasn't
that big!
If 'that' comes before a noun, it's a demonstrative pronoun (That fish); if 'that' stands alone, it's a substantive noun (That is); if 'that' refers back to a noun, it's a relative pronoun (books that have been sold); if 'that' can be omitted without changing the sentence's meaning, it's a conjunction (He said (that) he), and if 'that' can be replaced by 'so', it's an adverb (that big!).
Consider now,
Max: We have
six more papers to correct.
Pat: We don't have
that much to do!
'that' refers back to 'six more papers', so it functions as a demonstrative pronoun. 'much' functions as a noun, a substantive noun. Note, 'much' is the object of the verb 'have'.
In short,
EX: We don't have that (demonstrative pronoun) much (noun) to do.
Note, what Oxford calls a pronoun; i.e., much (pron.) is another way of saying, 'much' is a pro-form, "a substitution":
much stuff => that much stuff to do => that much to do
many papers => that many papers to do => that many to do
Above, both 'much' and 'many' substitute for 'much stuff' and 'many papers', respectively. They function substantively, just like pronouns;e.g., She = Mary : : much = the amount of six more papers.
All the best,