I think this is a scope problem. Since the event is triggered after I have added all the listeners num_markers has always been overridden by the next cycle in the loop.
Is there some way I can pass variables to the event function?
I tried this approach but it didn't for me. http://stackoverflow.com/questions/1841247/google-maps-event-listener-only-remembering-final-value-of-variable
            var map = new google.maps.Map(document.getElementById("map_canvas"),
            myOptions);
            var info_window = new google.maps.InfoWindow();
            var markers = [];
function load_markers() {
                var bounds_url = map.getBounds().toUrlValue();
                $.ajax({
                    url:'/retailer-markers?bounds='+bounds_url,
                    dataType: 'json',
                    success: function(data) {
                        for(i = 0; i < data.length; i++) {
                            var marker_pos = new google.maps.LatLng(data[i]['lat'], data[i]['long']);
                            //Every time the listener event is called this number is the length of the array
                            var marker_num = get_markers_count();
                            markers[marker_num] = new google.maps.Marker({
                                position: marker_pos,
                                map: map,
                                title:data[i]['Title'],
                                icon: image
                            });
                            google.maps.event.addListener(markers[marker_num], 'click', function() {
                                info_window.setContent('hello');
                                var pos = markers[marker_num].getPosition();
                                info_window.setPosition(pos);
                                info_window.open(map, markers[marker_num]);
                            });
                        }
                    }
                });
            }