OP, arithmetic is babby tier to explain. Fucking everyone since Plato knew this because it's intuitively obvious... for basic arithmetic.
Philosophy of mathematics gets interesting once you begin to try to explain the relation of higher mathematics and the world. Explaining what calculus is really about, limits or infinitesimals; explaining why shit like the Rieman zeta function has any correlative power to real world patterns even though it is an equation about a relation of purely mathematical abstraction (prime numbers) is what is really interesting. Your simple observation is just kek.
tl; dr: Mathematics begins as the science of quantitative empirical relations. That's not hard to see. What's hard to fathom is how what basically amounts to how relations of abstract relations that are not even quantitative, but purely abstract numeric shit, somehow has any correlation with reality. That's how I see it anyway.