Learn Portuguese (Portugal): Nós temos onze livros e doze canetas.

Temos is the present tense form of the irregular verb ter (to have) that matches nós (we). Whenever you want to say "we have", you will use temos.

Is it strictly necessary to include the word nós?

No, it is completely optional! Because the verb form temos can only mean "we have", the pronoun nós is often dropped in everyday speech. You can simply say Temos onze livros... and the meaning remains exactly the same.

Do the numbers onze and doze change based on whether the items are masculine or feminine?

No, numbers from three onwards (like onze and doze) do not change based on gender. They stay exactly the same whether you are counting masculine livros or feminine canetas. As a quick refresher, only the numbers one (um/uma) and two (dois/duas) change to match the gender of the noun.

How do I pronounce the word e (and) in this sentence?

The word e (meaning "and") is pronounced like the "ee" in the English word "cheese", but a bit shorter. Be careful not to confuse it with é (meaning "is"), which has an accent mark and sounds like the "e" in "pet".