I added 8 eggs when I should have only added 6...help
What else is in the recipe...? I'm assuming you don't want to increase the rest of the ingredients by 1/3? That's about my only suggestion. Otherwise, I'd just cross my fingers and go for it, depending on how important the cake is. Sorry that happened, it is so frustrating when you botch a recipe!
A lot of people add extra egg(s) to boxed mixes so I bet it will be just fine. Not knowing anything about the recipe it's hard to be more specific.