Given: ∣z−i∣=∣z+i∣=∣z−1∣
The given equation represents that z is a point such that it is equidistant from (0,1),(0,-1)&(1,0)
We know that in a triangle only circumcentre is the point which is equidistant from the three vertices.

ABC is a triangle. Hence its circum-centre will be the only point whose distance from A,B,C will be same.
So n(S)=1