I think that's conflating distinct concepts. Numbers aren't a physical property. Whether or not they're a fundamental concept is largely a philosophical question.
Meanwhile temperature (for example), while physical, is a statistical property of a macroscopic system. It isn't fundamental - rather it's an abstraction over a (very) large set - but it is nonetheless a quantification of physical characteristics. Whereas a number, for example 2 rocks, is not quantifying any physical property of the individual rocks themselves.
The existence of a rock is definitely a physical property, and thus is the number of rocks in a collection a physical property, at least as much as temperature is it for a gas.
Numbers aren't "existence" they're a property of an abstract set. The observer draws a fairly arbitrary mental line around a cluster of objects. Whether the set numbers two, three, or four cannot be determined by observing one of the rocks in isolation.
Fundamental physical properties don't depend on other members of an arbitrary set as determined by an observer.
> at least as much as temperature is
But here I've been explicitly claiming that temperature isn't. It is a statistical abstraction over a physical property but it is not itself a physical property in any inherent sense. That's ... kind of the entire point I've been trying to make about an entire class of concepts.
Why would you need to observe anything in isolation for it to be a physical property? This is the proposition that I find absurd. It’s like saying that a hydrogen atom isn’t really a hydrogen atom, but just an arbitrary set of nucleons.