Why den and hans both in sentence
I had the same question. Initially, I had put "Det er hans sjuende hesten."
Why not "sin" ?
"Sin" can only refer to an object owned by the subject of the sentence. "That" refers to the horse, not to its owner.
Is 'syv' ever used for seven?
Yes, bot "sju" and "syv" are quite common. You can use either, but be consistent about it.
Is det used with numbers rather than den?