The best way to do this would be to create a "Quantity" question instead of using the registration levels.
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To create a quantity question, please take the following steps:
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1). Visit the "Form Questions" tab at the form level.
2). Click on "Add Local" under the "Your Information" section or the "Additional Questions" section. This will then allow you to create a question from scratch.
3). You will then need to choose the type of question, so choose the "Quantity" option.
4). Be sure to include an internal name and label as well as an optional fee that will be multiplied by the QTY that the user chooses.
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*Please note, if you use this method, you will then need to remove the registration level that you had created for the ticket sales.