So you hold the Fictionalism view and reject Platonism. That's fine. This stuff has been debated ad nauseam and we're not going to break any new ground here.
> It's like saying unicorns exist. They're a shared fiction. (A highly useful fiction, to be sure)
I will mention that dogs are also a useful shared fiction. We assign the label dog to a perception of a configuration of atoms that we think satisfy the criteria for what a dog is. So I'd say that numbers are more like dogs than unicorns, in that they are an abstraction that is used by us in practice to map to perceptions of physical objects.
I've noticed people in this thread are getting actual, physical things confused with concepts and mental representations of things.
Confusing the map for the territory.