You can specify the quantity of the grocery to be purchased by simply adding it at the beginning when you add a grocery, eg 2 milk. The next time you add a grocery, the app will remember if you specified a quantity the last time and automatically add that quantity to the grocery.
When you swipe a suggestion to the grocery list, Grosh will automatically enter a suggested purchase quantity based on your past behaviour. You can of course change this by displaying the grocery details and tapping – or + at the top.