Jonathan Wagner of Scribble Maps fame has created a demo Google Map displaying 100,000 markers on one map. As you can see from the screenshot above that is enough markers to completely obscure the map tiles beneath.
Amateur map developers should of course not try this at home without wearing safety goggles and without the supervision of a qualified Google Maps Developer.
Usually if you want to place a very large number of markers on a map you will want to use a marker clustering solution such as Mapeed. However, if you are feeling brave, Jonathan has been kind enough to provide the source code for his MillionMarkerMap.