Zero is not a number in the set of positive integers, which are the counting numbers dating from antiquity.  It had to be invented and is still to my mind something we call a number but isn't quite, since it is not used to count anything but only to indicate absence of anything to count.

It is of course essential for modern mathematics, but mathematics is divorced in many ways from reality and is abstract, maybe and maybe not the basis of existence (one can find quotes on both sides of this).