Learn Croatian: Očekujemo puno volontera. - We expect a lot of volunteers.

The infinitive is očekivati (to expect). This belongs to the special group of verbs ending in -ovati or -ivati that you are learning now. To form the present tense, you drop the -iva- and replace it with -uj- before adding the usual endings. So, oček-iva-ti becomes oček-uj-emo (we expect).

Why is the noun volontera in this specific form?

It is in the genitive plural case because it comes after puno (a lot). In Croatian, whenever you use a word expressing a quantity like puno or malo (a little), the noun that follows must be in the genitive case. Since we expect multiple volunteers, we use the genitive plural, giving us volontera.

Is there another word I could use instead of puno?

Yes, you can use mnogo (many / a lot). Both puno and mnogo mean the same thing here and both require the following noun to be in the genitive case. Puno is just a bit more common in everyday spoken language.

Why isn't there a word for "we" in this sentence?

In Croatian, the pronoun mi (we) is usually dropped because the verb ending -mo in očekujemo already makes it perfectly clear who is doing the expecting. You would only add mi if you needed to add strong emphasis.