Learn Croatian: Umnožit ćemo ovaj primjerak dva puta. - We will reproduce this copy two times.

After the number 1 and ordinal numbers (like first, second), you use put. For all other numbers (like 2, 3, 5) and quantifiers (like više), you must use the crystallized genitive form puta. Since the number here is 2 (dva), we use puta.

Why is the verb written as Umnožit instead of the full infinitive Umnožiti?

In the Croatian future tense, when an infinitive ending in -ti comes immediately before the auxiliary verb (like ćemo), the final -i is dropped. This makes pronunciation smoother, so umnožiti ćemo is written and spoken as umnožit ćemo.

What case is ovaj primjerak in, and why didn't it change from its dictionary form?

It is in the accusative case because it is the direct object of the verb umnožiti. For masculine nouns that represent non-living things (inanimate), the accusative form is identical to the nominative form, which is why it stays ovaj primjerak rather than taking an ending.

Could I use the word kopija instead of primjerak here?

Yes, kopija is a very common word for a direct duplicate or photocopy. However, primjerak emphasizes that it is a specific 'specimen' or individual item from a batch (like a single physical copy of a printed book or document), which is very natural when talking about reproducing an original.