Like they said, that's the minimum. Usually those are efficient enough so you can have a single window unit with a pane in the middle, but quadruples are common

Like they said, that's the minimum. Usually those are efficient enough so you can have a single window unit with a pane in the middle, but quadruples are common too. Unfortunately, having more panes of glass does not automatically mean you get more energy efficiency. The triples with very good insulation and coatings (eg. heat rejection) can be better than old quadruples. My parents have quadruple windows from the 80's, and energy efficiency is much better in a modern triple single unit than with the two doubles they have.