function initialize() {
var goo = google.maps,
map = new goo.Map(document.getElementById('map'), {
zoom: 3,
center: new google.maps.LatLng(0,0),
noClear:true
}),
markers = [],
randLatLng = function(){
return new goo.LatLng(((Math.random() * 17000 - 8500)/100),
((Math.random() * 36000 - 18000)/100));
},
count = document.getElementById('count');
map.controls[goo.ControlPosition.TOP_CENTER].push(count);
for (var i = 0; i < 500; i++) {
var marker = new google.maps.Marker({
position: randLatLng(),
icon: 'https://chart.googleapis.com/chart?chst=d_bubble_text_small&chld=bbT|'+i+'|FF8|000'
});
(function(m){
var listener=google.maps.event.addListener(m,'map_changed',function(){
if(this.getMap()===map){
count.firstChild.data=parseInt(count.firstChild.data,10)+1
google.maps.event.removeListener(listener);
}
});
}(marker));
markers.push(marker);
}
var markerCluster = new MarkerClusterer(map, markers);
}
google.maps.event.addDomListener(window, 'load', initialize);
html,body,#map{height:100%;margin:0;padding:0;}
#count{background:tomato;padding:8px;font:bold 1.3em bold Monospace !important}
#count::after{content:' of 500 markers added to map'}
<div id="map"><div id="count">0</div></div>
<script src="https://maps.googleapis.com/maps/api/js?v=3"></script>
<script src="https://googlemaps.github.io/js-marker-clusterer/src/markerclusterer_compiled.js"></script>