You can edit the number of addons while adding through 'Add One-Time Charges and Addons' option. There is an option to edit the price per unit, and quantity for unit-based addons whereas for flat fee addons, you will be able to edit only the amount and not the quantity.
Once you go to subscription details page -> click on 'Add One-Time Charges and Addons'
Click on 'Add One-Time Addon' search your unit-based addon-> Configure the timeline -> Click on 'Add'. You will see the below page where you can edit the quantity.
This feature is available in both Leap and Classic UI.