At first. Whats rule that defines which product should be visible with qty?
Specific category? Specific Attribute set? Specific attribute value?
Then on cart check those rules for every entity.
By default phtml that displays qty is here:
But it may be somewhere else (depends on theme and project custom modules).
Search by qty.
I mean, create own module with logic that return false or true for qty input, override this phtml with block. Using block, call function that return bool for qty for specific product and show or not this input depends on bool.