Learn Portuguese (Portugal): Eles moram no número cinquenta.

Yes, the u in cinquenta is pronounced! Unlike some other Portuguese words where qu sounds like a hard k, here it makes a kw sound. This is the cardinal number for fifty.

Why does the sentence use no for the English word at?

In Portuguese, you use the preposition em to talk about locations and addresses. Because número is a masculine word, em combines with the word for the, which is o, to make no. So, you are literally saying in the number fifty.

Where does the verb moram come from?

It comes from the regular verb morar, which means to reside. Since the subject is eles, which means they, we drop the -ar ending and add -am to make moram.

Could I use the verb viver instead of morar in this sentence?

Yes, you could say Eles vivem no número cinquenta. Both morar and viver can mean to live in a place. Morar specifically means to reside at an address, while viver is also used for being alive, but for addresses they are completely interchangeable.