What set of number does -1 belong?

Answer 1

To any set that contains it!

It belongs to {-1},

or {45, sqrt(2), -1, pi, -3/7},

or all whole numbers between -43 and 53,

or square roots of 1,

or negatives of counting numbers,

or integers,

or rational numbers,

or real numbers,

or complex numbers,

etc.