。
My father smokes at least a pack a day.
Literal
Father [topic-は] one-day [per-に] one-box [at least-は] tobacco [object-を] smokes [explanatory-んです].
The は after 一箱 is not a topic marker but a minimum-emphasizer: 'at least one box.' This subtle use of は to set a floor is distinct from its topic and contrast functions. ~んです wraps the statement as an explanation, as if in response to a question.