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1.
To get a lot of something that is good. Used to express your feeling of joy because you just aquired more of something.
Guy 1: Dude, you are totaly taking advantage of their endless nachos promotion.

Guy 2: Quantity~~ (face beaming with joy)
by Yip July 23, 2004
 
2.
An expression used to indicate the awesome nature of a number of things, such as an event, physical object, or situation.
The delicious chocolate lava cake was quantity, as was the party on Saturday night.
by MaGi_TekKx September 15, 2008