Unfortunately it's not very easy. You could create your own custom marker based on the OverlayView class (an example) and put your own HTML in it, including a counter. This will leave you with a very basic marker, that you can't drag around or add shadows easily, but it is very customisable.
Alternatively, you could add a label to the default marker. This will be less customisable but should work. It also keeps all the useful things the standard marker does.
You can read more about the overlays in Google's article Fun with MVC Objects.
Edit: if you don't want to create a JavaScript class, you could use Google's Chart API. For example:
Numbered marker:
http://chart.apis.google.com/chart?chst=d_map_pin_letter&chld=7|FF0000|000000
Text marker:
http://chart.apis.google.com/chart?chst=d_map_spin&chld=1|0|FF0000|12|_|foo
This is the quick and easy route, but it's less customisable, and requires a new image to be downloaded by the client for each marker.
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