To add more than one of the same modifier to a main product:
Select your main product e.g. Milk Tea
Select a modifier and you'll see '1' at the top right of the modifier.
Tap and hold the modifier to input new modifier quantity e.g. 2.
When confirmed, you'll see '2' reflected on the top right of the Pearl topping selected.
It will also be reflected in the Current Sale, as well as on your receipt.