How to choose between 'much' and 'many'?

GMAT sentence correction questions sometimes ask candidates to choose between 'much' and 'many'.

How does one decide which word to use?

In spoken English, we might unknowingly be using these words incorrectly, so knowledge of the underlying grammar rules is necessary. 

Example 1

Take a look at the following two sentences.

(A) Do you need that much milk?

(B) Do you need that many liters of milk?

As we can see, the noun in option (A) i.e. milk can only be used in the singular. Milk cannot be used with a number.

Whenever this is the case, we cannot use 'many'. We have to use 'much'

So option (A) is correct

In option (B), we are not just referring to 'milk' as a commodity. 

Instead, we are referring to 'liters of milk', which can have both a singular and a plural form.  

For instance, we can say "A liter of milk" or we can even say "10 liters of milk"

Whenever this is the case, we cannot use 'much'. We have to use 'many'

So option (B) is also correct

Example 2

Now take a look at the following two sentences.

(A) That does not make much sense to me.

(B) The sense of touch is one of the much senses that humans can experience.

The noun 'sense' can be countable or uncountable. In more general - commonly used contexts - the plural form will also be sense. 

However, in more specific contexts, the plural form can also be senses e.g. in reference to various types of senses or a collection of senses.  

In option (A), we are using 'sense' in the more general, commonly used, context, namely:

the ability to behave in an intelligent way and make good decisions

Hence it can take the singular form only.

Therefore, use of 'much' is appropriate here and option (A) is a good sentence

In option (B), however, we are using 'sense' to mean:

one of the natural abilities that most people have to see, hear, smell, taste, and feel things, known as the five senses

Hence it can take both the singular and plural forms.

Therefore, the use of 'much' is incorrect  in option (B).  The correct sentence should read:

(B) The sense of touch is one of the much many senses that humans can experience.