Learn Croatian: Naš akvarij sada ima stotinjak pingvina. - Our aquarium now has about a hundred penguins.

It means "about a hundred." It is formed by adding the suffix -jak (a variation of the -ak suffix) to the base word for a hundred (stotina). You can use this suffix on numbers to express an approximate amount, similar to saying "around" or "some" in English.

Why is the word for penguins (pingvina) in the genitive plural?

Because it comes right after stotinjak. In Croatian, words that express an approximate quantity using the -ak suffix act like nouns and always require the thing you are counting to be in the genitive plural. You can think of the literal translation as "about a hundred OF penguins."

What case is stotinjak in here, and why didn't its ending change?

It is in the accusative case because it is the direct object of the verb ima (has). Even though it is in the accusative, its form doesn't change from its base dictionary form. This is because stotinjak behaves as an inanimate masculine noun, and for those, the nominative and accusative forms are identical.

Why do we use naš instead of naša or naše for "our"?

Akvarij is a masculine noun, which you can tell because it ends in a consonant. Because "our aquarium" is the subject of the sentence, the possessive pronoun must be in the masculine singular nominative form to match it, which is naš.