Great question. Numbers, in maths, come from axioms. In other words assumed to exist. So, the incompleteness theorem would not apply to them, only to the set of all statements that derive from the starting list of axioms. It doesn't actually matter which axioms you start with either. 1+1=2 is a good example.

Actually, it is debatable whether even God could know something that cannot be known, and this is where, if you believe that maths and number are pure and existed without the hand of God, a logical problem occurs and this sent Gödel mad, poor thing.