According to the Avatar Wiki, Aang and Katara's first two children -- named Kya (for Katara's mother) and Bumi -- were non-benders.
Kya is a waterbender, Bumi ist the only non-bender and some fans are already speculating that he's Amon. I hope not because that would be really lame.
We know there are more than four now. Can the Avatar metal-bend as well?
No, there are still only four elements, metalbending is earthbending and of course the avatar can metalbend, every earthbender can theoretically learn it, it's not its own bending style, just an advanced form.