"That is his seventh horse."

Translation: Det er den sjuende hesten hans.

[SAM_Kh]

Why den and hans both in sentence

[Heithr]

I had the same question. Initially, I had put "Det er hans sjuende hesten."

[mazeido]

Why not "sin" ?

[Luke_5.1991]

"Sin" can only refer to an object owned by the subject of the sentence. "That" refers to the horse, not to its owner.

[coolcats4]

Is 'syv' ever used for seven?

[Deliciae]

Yes, bot "sju" and "syv" are quite common. You can use either, but be consistent about it.

[KariThorsdatter]

Is det used with numbers rather than den?