There is no single word meaning quantity-related. In theory that word would be "quantitative", but that has evolved to have a slightly different, more specific meaning.
I can answer the second part of your question. Adjectives made up of two words like that are called compound adjectives. When they are used before a noun, the hyphen is generally optional, but is mandatory when needed to resolve ambiguity. For instance in "a heavy metal detector" it's unclear what "heavy" applies to; it should be "a heavy-metal detector". Leaving out the hyphen in "a quantity-related discount" would look strange. It's never wrong to use a hyphen before a noun, so just always using one is a good rule.
(Edit: I should have said it's never wrong as long as the compound adjective represents a single, separate idea. For example "red-light district" is wrong; "red light district" is a single concept here that should not be broken into components. But "The reaction is carried out under red-light conditions" is correct, for example)
When a compound adjective is NOT before a noun the hyphen should USUALLY be omitted, e.g. "a high-stakes game" but "The game was high stakes". But there are (surprise!) exceptions, for instance "The price is all-inclusive". I'm not sure if there are rules for this or if it's just case by case. I would hyphenate in "The discount was quantity-related". I think if the first word in the compound is not an adjective (ALL-inclusive, QUANTITY-related) it should always be hyphenated.