These are all used to describe or measure quantity. The difference is whether what is being measured is countable or not.

量 - How to show amount (weight, volume, proportion). Used for uncountable items. 

数 - How to show number. Used for countable items.

数量 - A general way of saying quantity or quality (or both) without having to be specific about which.

分量 - How to show the measure of a fixed non-countable quantity. 

個数 - How to show the measure of a fixed countable quantity. 

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When describing how much food is on one's plate in plain terms, one would use 量, while the number of sausages on one's plate would be 数. [A lot of food vs. Many sausages]

When determining set serving sizes/quantities: For mashed potatoes one would use 分量, while number of sausages would be 個数. [Amount of mashed potatoes to be served vs. Number of sausages to be served]

When describing the serving sizes/quantities in general (without being specific), one would use 数量. [Amount and number]  
Also, when ordering online, often the field for quantity/number to be ordered uses the default 数量, as it would be too much trouble to tailor each field to each individual product.

In colloquial terms, 量 and 数 are the most commonly used.