Does quantity exist in the universe?

But could you define it in any other way?

yeah, dude what if... *passes blunt

Air is a gas containing molecules such as N2, O2, CO2 and so on

yeah but what about space? What about when you remove the air under a big glass bell? Pressure goes down but is it "empty"?

Unlike in solids, there's much more "space" between gas particles, which is why gases can be compressed. What this empty space consists of, you're better off asking someone else or looking up yourself.

"Nothing" is nowhere. As of definition it does not exist.

There are bodies and void. But the "boundary" in-between is infinitely fine.

Bodies are in the void.
Bodies pass through the void instantly, as there is no resistance in the void.

The boundary of bodies to the void, is saturated with infinitely minuscule bodies, as a film around it, expressing its surface:

"Again, there are outlines or films, which are of the same shape as solid bodies, but of a thinness far exceeding that of any object that we see. For it is not impossible that there should be found in the surrounding air combinations of this kind, materials adapted for expressing the hollowness and thinness of surfaces, and effluxes preserving the same relative position and motion which they had in the solid objects from which they come. To these films we give the name of “images” or “idols.” Furthermore, so long as nothing comes in the way to offer resistance, motion through the void accomplishes any imaginable distance in an inconceivably short time. For resistance encountered is the equivalent of slowness, its absence the equivalent of speed." - Epicurus letter to Herodotus

The films are of infinite complexity and fineness, and as such; with infinitely infinitesimal resistance, it moves both inside and around whatever it expresses:

"We must also consider that it is by the entrance of something coming from external objects that we see their shapes and think of them. For external things would not stamp on us their own nature of color and form through the medium of the air which is between them and use or by means of rays of light or currents of any sort going from us to them, so well as by the entrance into our eyes or minds, to whichever their size is suitable, of certain films coming from the things themselves, these films or outlines being of the same color and shape as the external things themselves. They move with rapid motion; and this again explains why they present the appearance of the single continuous object, and retain the mutual interconnection which they had in the object, when they impinge upon the sense, such impact being due to the oscillation of the atoms in the interior of the solid object from which they come. And whatever presentation we derive by direct contact, whether it be with the mind or with the sense-organs, be it shape that is presented or other properties, this shape as presented is the shape of the solid thing, and it is due either to a close coherence of the image as a whole or to a mere remnant of its parts. Falsehood and error always depend upon the intrusion of opinion when a fact awaits confirmation or the absence of contradiction, which fact is afterwards frequently not confirmed or even contradicted following a certain movement in ourselves connected with, but distinct from, the mental picture presented—which is the cause of error." - Epicurus letter to Herodotus

How do they define quantity? What's their argument?

Usually a quantity is understood to be a real number along with a unit. However, the units you use are somewhat arbitrary, so the number itself only has meaning when the quantity is dimensionless. So for example you could say that you're going 60 mph or ~100 km/h, it's equivalent, and the number itself is not "real." However, if you compare your speed to someone else's the ratio itself will be real, in the sense that it doesn't depend on the units you use.

If you mean that numbers themselves don't "exist" then yes, people have speculated that the standard continuum of real numbers is built from something more fundamental. But this is still a hypothetical; no one has provided a model of physics that doesn't fundamentally use the real numbers in some way.