This is a pretty easy modification, and only requires one small change.

First, open up the Main Index Template for editing (index.php) through the Wordpress administration "Theme Editor".

Search for the following line:

  <?php $my_query = new WP_Query('category_name=products&showposts=12'); while ($my_query->have_posts()) : $my_query->the_post(); // check for product large image

Locate in the line where it says "showposts=12".  Change the number "12" to whatever number you like.

NOTE:  It's usually best to keep the number a multiple of 3, since the design layout is optimized to fit 3 thumbnail images across.  (ie. 3, 6, 9, 12, 15, 18, etc.) 

Save the file.

That's it.

No… Market has advanced since then.

You can now change the number of thumbnails shown through the "Market Theme" options settings area of your Wordpress administration dashboard.