How would you say, "He does not have one car. He has two."?

Bonjour Diane,

If you wanted to emphasise the number of cars he has, you could indeed say:
"Il n'a pas une voiture. Il en a deux.",
and even more colloquially, you could say:
"Il n'a pas une voiture, mais deux !"

Thank you for pointing out that case, we added this distinction to the existing lesson:
https://french.kwiziq.com/revision/grammar/un-and-une-become-de-or-d-in-negative-sentences-indefinite-article

Merci et à bientôt !