And also the zero appears to be automatically "added" when you want to edit already published events in "Maximum Group Size" and "Send New Registration Notification Emails To" fields which was properly filled out when we published the new event.
One thought is that maybe you have CB integration enabled without having it installed. Beyond that, not sure. Has to be something specific to your site. Please log a support ticket and we'll check it out.
Turns out he had some bad data in his config database table. Simply re-saving the DT Register configuration fixed the issue. Anytime you upgrade, be sure to re-save the config to make sure all changes and new options have been saved properly. Thanks.