The custom fields should be listed in the order you defined them. It's a little unclear based on what you've specified above how you have things set up. Have you defined multiple sponsors in a single custom field, or are you using multiple custom fields? You've also indicated the key is named either 'sponsorship' or 'sponsor'. You made mention of the widget, but you provided an example using the template tag.
I'm going to assume you are using the key 'sponsorship' and that you are defining multiple custom fields of that key for a single post, one for each sponsor.
In that case, you'd want to use the following snippet:
<?php echo c2c_get_custom( 'sponsorship', 'My Most Recent Sponsors:', '', '', ', ' ); ?>
You example code won't list more than one at a time, since the plugin by default only lists one. You have to define the argument that indicates what to put between items in order to list multiple items. My snippet simply puts a comma and space between each listed sponsor.
If I've misunderstood what you're trying to do, please clarify.