Learn English – Can we say “He didn’t change very many”

As little as, as much as, as few as and as many as are idiomatic phrases.

The definitions in Merriam-Webster dictionary are:

• As little as: used to suggest that a number or quantity is surprisingly small
• As few as: used to suggest that a number or amount is surprisingly small
• As many as: used to suggest that a number or amount is surprisingly large

But

• As much as: used to say that two things are equal in amount or degree, or that an amount is as large as another amount; other meanings are "almost but not quite" and "even though".

As you can see, "as little as" is almost the same as "as few as", but the former can be used for a quantity while the latter can be used for an amount (see this question for the difference). These two are interchangeable in some cases. But all the examples you provided are related to continuous quantities, so "as few as" is not suitable. As a suitable example, you could say "In my math class there are as few as two girls".

However, "as much as" does not carry the same meaning - it is typically used in different contexts (which makes it much more common; see these Ngrams). Using "as much as" instead of "as many as" (as in your third example) may be considered correct, but it will likely create some confusion when the sentence is read, so I would suggest using "as many as" instead.

Good point. It's not short for a number of things; you could think of it as short for an amount of work or stuff to do. (These are mass nouns.)

• I have a lot of stuff to do.
• I have so much stuff to do.
• I have very little work to do.

Etc. For whatever reason, "I have many/few to do" is not the expression.