The "quantity" field... do you mean round count?
There should be a dismiss keypad button on the top right of the app.
The update issue is with Gun Log, not Gun Log +P.
The view scrolls, so if the field is covered, just drag it into view.
The round count is how much you have on hand when you start tracking usage with the app. You don't decrement the round count when you shoot some, unless you didn't track it with a firing set.