Mere addition, as mentioned by the other user is the primary one I was referring to, but breaking different approaches to utilitarianism only requires one to take them to their extremes.
If average welfare of humans is all that matters, then one happy human living alone in the universe is the equivalent of a million happy humans.
If sum of "welfare" is all that matters, then you can argue an exceptionally large number of people being tortured indefinitely is better than a happy person.
