To Drop pins at several locations u have to try this code:
You need to put all those location latitude and longitude in an array and call this function  everytime after incrementing the num value 
-(void)Mapview
{   NSMutableArray* annotations=[[NSMutableArray alloc] init];
    CLLocationCoordinate2D theCoordinate1;
    theCoordinate1.latitude = [latit[num]doubleValue];
    theCoordinate1.longitude = [longit[num]doubleValue];
    Annotation* myAnnotation1=[[Annotation alloc] init];
    myAnnotation1.coordinate=theCoordinate1;
    [map_view addAnnotation:myAnnotation1];
    map_view.showsUserLocation = NO;
    [annotations addObject:myAnnotation1];
    MKMapRect rect = MKMapRectNull;
    for (id <MKAnnotation> annotation in annotations) {
        MKMapPoint annotationPoint = MKMapPointForCoordinate(annotation.coordinate);
        MKMapRect pointRect = MKMapRectMake(annotationPoint.x, annotationPoint.y, 0, 20);
        if (MKMapRectIsNull(rect)) {
            rect = pointRect;
        } else {
            rect = MKMapRectUnion(rect, pointRect);
        }
    }
    map_view.visibleMapRect = rect;
}
You can edit this code by adding leftcallout acessoryview  or modify the contents in callout view .
- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation{
        map_view.showsUserLocation= NO;
        // if it's the user location, just return nil.
        if ([annotation isKindOfClass:[MKUserLocation class]])
            return nil;
         MKPinAnnotationView* pinView = (MKPinAnnotationView*)[mapView dequeueReusableAnnotationViewWithIdentifier:@"AnnotationIdentifier"];
        if(pinView == nil)
        {  
            pinView = pinView=[[MKPinAnnotationView alloc]
                               initWithAnnotation:annotation
                               reuseIdentifier:@"currentloc"] ;
            pinView.centerOffset = CGPointMake(0,60);
            pinView.animatesDrop=YES;
            pinView.canShowCallout=YES;
          }
        return pinView;
}
To Remove the pins you can use this code 
        [self.map_view removeAnnotations:map_view.annotations]; 
        (or)
        [self.map_view removeAnnotations: [self.map_view.annotations filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"!(self isKindOfClass: %@)", [MKUserLocation class]]]];