Question

When a user zoom at the maximum level that Mapkit permits I want to show all the pins.
How to do that?

Answer 1

By listening to the longitudeDelta of the maps span in the regionDidChange method.
Minimum longitudeDelta seems to be 0.000858 by testing and logging it.

- (void)mapView:(MKMapView *)aMapView regionDidChangeAnimated:(BOOL)animated{
    NSLog(@"Deltalong: %f", aMapView.region.span.longitudeDelta);
    if (aMapView.region.span.longitudeDelta <= 0.000858){
        //add your annotations
    }
}