Hello,
To reach this quantity option in these 2 different context, and allow to define product quantity, follow me step by step :
For the Checkout :
1. From Main HikaShop configuration, go to Checkout tab
2. In Checkout Workflow part, find every Cart block (if you use more than one of course)
3. To allow your cart part to be editable you must set it on "Read only"
For your Cart :
1. Go to Extensions => Modules
2. Use the search input with "Cart" in order to find the relative module
3. Click on his title to edit, and go to HikaShop options tab
4. In Data Display part, you will find many option, and among them Product quantities option.
Hope this will help you to fit your needs.
Regards