The phrase has been popular in the US defense community since the 1980s, sometimes acknowledging it as a US coinage, but often misattributing it to Clausewitz, Lenin, Stalin, and Brezhnev, but mostly to Stalin.
The general principle that quantity begets quality is a key tenet of the Marxist theory of dialectical materialism, as formulated by Marx and Engels, phrased as the law of the passage of quantitative changes into qualitative changes. This in turn is attributed to Hegel (Science of Logic), who in turn attributes it to Ancient Greek philosophers, notably the paradox of the heap Eubulides: a quantitative change in the number of grains of sand leads to a qualitative change in being a heap or not. While Marx and Engels are quoted by various Marxist and communist authors, including Stalin, this formulation is not found in their work or in English translation. (Re: “Quantity has a quality all its own” source?, Tim Davenport, H-Russia, April 5, 2010)