Well you can think of mathematics as a thought experiment to derive a maximally consistent logical structure from nothing but a agreed-upon set of axioms.

Our current knowledge of mathematics is based on a axiom set called ZFC which is predominantly a bunch of set theoretic axioms, including ones guaranteeing that you can construct natural numbers by counting elements.
So you see even the most complex mathematics can be -depending on your stance- very real or very abstract because fundamentally it requires little more than the ability to count objects, all the complicated details can be derived and built up from there.