In my project I used the following, inside the mapView:layerForAnnotation:
method of the map delegate:
if ([annotation.annotationType isEqualToString:@"RMClusterAnnotation"]) {
UIImage *clusterImage = [UIImage imageNamed:@"foo.png"];
RMMarker *newMarker = [[RMMarker alloc] initWithUIImage:clusterImage];
// this is how I managed to count the annotations inside a cluster
NSInteger annotationsInCluster = [((RMQuadTreeNode *)annotation.userInfo).annotations count];
// you can add a label to the annotation with the number of clustered annotations
[newMarker changeLabelUsingText: [NSString stringWithFormat:@"%i", annotationsInCluster]];
return newMarker;
}
Hope this works for you!