There are many different words used to describe a particular quantity of something. Usually the word is joined to the noun it describes with ‘of’.

Containers (e.g. a bag) and contents (e.g. of shopping):

With uncountable nouns:

When we use uncountable nouns (e.g. advice), we sometimes want to talk about one of something. We cannot say an advice or one advice, but we can use certain words to make these nouns singular: a sheet of paper (= one piece of paper) or a slice of bread (= one piece of bread). We can actually use the word piece with many nouns:

You can use piece with some abstract nouns, e.g. a piece of information, a piece of luck.
The most common partitive is a bit, which is informal and used with many nouns. It usually means a small amount but can be quite general. It can be used with the examples above, and more: a bit of butter, a bit of time, etc.

A pair of …

Some nouns have two parts, e.g. trousers (two legs) and shoes (left and right). You can use a pair of to specify the number, e.g. a pair of skis, two pairs of shoes, three pairs of tights.

Groups of things:

Gang has a negative meaning: it suggests a group of young people who may cause trouble.