"The cat eats its mice."
Translation:Katten spiser sine mus.
Based on sin/sit or sine.
"sine" indicate the objective "mus" in that sentence is plural.
Singular --> Katten spiser sin mus Plural --> Katten spiser sine mus
One more example, Singular --> Han læser sin bog Plural --> Han læser sine bøger
So, you can find out whether the objective is singular or plural by looking sin/sit or sine in front of it. "Sine" indicates the objective is plural