A discount directly reduces what each unit contributes when the variable cost stays the same. That is why the sales you need can grow much more than the discount percentage.
An example with numbers
You sell at $50 with a $30 variable cost. Each unit contributes $20. After a 10% discount, the price drops to $45 and the contribution to $15. To match 60 original sales you would need 80 sales on promotion.
Try it with your numbers
Try the discount you have in mind and compare the units needed with the sales increase you realistically expect.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.