If we have multiple polygons and we want to calculate the minimum bounding rectangle, how can we do this?

In the native iOS it's done http://stackoverflow.com/questions/1303265/algorithm-for-determining-minimum-bounding-rectangle-for-collection-of-latitude

To get min bounding rectangle of a bunch of geometries, you should union them together using the geometry engine - 10.2.5: AGSGeometryEngine Union

You can then use the envelope of the resulting geometry