You need to keep track of some offset in the class instantiating these:
float x_offset =0;
float y_offset =0;
then in your code:
Vector3 position = transform.position;
position.x += x_offset;
position.y+=y_offset;
GameObject showIcon = Instantiate (showAppIconPrefab, position, transform.rotation) as GameObject;
x_offset+= SomeVariableYouSet;
if (x_offset > SomeValue) // so we don’t scroll off edge of screen
{
x_offset = 0;
y_offset+= SomeValueYouSet; //this starts a new line
}