Learn Italian: Abbiamo comprato due frigo spaziosi per il ristorante. - We bought two spacious fridges for the restaurant.

Frigo is a clipped version of the longer word frigorifero. In Italian, nouns that are shortened like this are invariable, meaning their endings do not change in the plural. So, one fridge is un frigo and two fridges are due frigo.

If frigo does not change in the plural, how do we know it is plural here?

You can tell by the words around it! Here, the number due and the plural adjective spaziosi make it clear that we are talking about more than one fridge. If we used a definite article, that would also change to the plural, as in i frigo.

Why does the adjective spaziosi end in -i if frigo ends in -o?

Even though the noun frigo is invariable and keeps its -o ending, it is still grammatically a masculine plural noun in this sentence. Adjectives must always agree with the true gender and number of the noun they describe, so the masculine singular spazioso becomes the masculine plural spaziosi.

Could I use the full word frigoriferi instead in this sentence?

Yes, you can absolutely say due frigoriferi spaziosi. The shortened version frigo is just much more common in everyday spoken Italian, similar to how we say fridge instead of refrigerator in English.