At the moment when you create a Mystery Code, it takes into account all of the Mystery Boxes you have at that particular time. The Mystery Code itself is valid for 24 hours and once you successfully reveal your Mystery Boxes on the EARN'M dApp, you will be able to create a new Mystery Code.
But there are some special scenarios - for example, if you have 10 Mystery Boxes when you create your code but before you get to reveal it on the dApp you get another Mystery Box through an email or your Mode Earn Club membership. In that case now you have 11 Mystery Boxes, however, you will see on your Mystery Code creation page that your code only includes 10 boxes.
If you want to make sure that you reveal all 11 Mystery Boxes together, you’ll have to wait for 24 hours until the code you created with the 10 boxes is no longer valid. That’s when you’ll be able to create a new code that will then take into account all 11 boxes. But otherwise, you can simply reveal the 10 Mystery Boxes, come back to the app and create a new code for the 1 more Mystery Box as well. This will cost you an additional cent or so for the transaction fee, but it’s really not worth the mention.