Nos faltan dos sillas para la terraza.

Because faltar and necesitar do not work the same way.

• Necesitamos dos sillas = We need two chairs
• Nos faltan dos sillas = literally, Two chairs are missing to us

So in Spanish, faltar often expresses the idea of lacking something rather than directly needing it. It is a very common way to say that you are short of something.

This sentence sounds a bit like:

• We’re two chairs short for the terrace
• We still need two chairs for the terrace

Because the verb agrees with dos sillas, which is plural.

In this structure, the thing that is lacking is the grammatical subject:

• Falta una silla = One chair is missing
• Faltan dos sillas = Two chairs are missing

So:

• dos sillas = plural
• therefore faltan = plural verb

Nos means to us or for us.

It is an indirect object pronoun. In this sentence, it shows who is affected by the lack.

So:

• Nos faltan dos sillas = Two chairs are missing to us / We are lacking two chairs

Other examples:

• Me falta dinero = I’m short of money
• Te faltan cinco euros = You’re five euros short
• Les falta tiempo = They don’t have enough time

Yes. That is one of the biggest differences from English.

In English, we say:

• We need two chairs

But with faltar, Spanish is structured more like:

• Two chairs are lacking to us

So the subject is dos sillas, and that is why the verb is plural: faltan.

A good test is to switch the number:

• Nos falta una silla
• Nos faltan dos sillas

The verb changes according to silla/sillas, not according to nos.

Because Spanish normally does not use an article before a number in this kind of sentence.

So:

• dos sillas = two chairs not
• las dos sillas = the two chairs

If you said las dos sillas, you would be talking about two specific chairs already known in the context:

• Nos faltan las dos sillas que compramos ayer = We’re missing the two chairs we bought yesterday

But in your sentence, it just means two chairs, in a general counting sense.

Para la terraza means for the terrace: the chairs are needed for use on the terrace or as part of the terrace setup.

Using para focuses on purpose or destination.

• Nos faltan dos sillas para la terraza = We need two more chairs for the terrace

If you said en la terraza, that would sound more like location:

• Faltan dos sillas en la terraza = There are two chairs missing on/from the terrace

So:

• para la terraza = for the terrace
• en la terraza = on/in the terrace area

Yes, absolutely. That is very natural too.

Both mean roughly the same thing:

• Nos faltan dos sillas
• Nos hacen falta dos sillas

Often, hacer falta can feel a little more explicitly like to be needed, while faltar can feel a little more like to be lacking. But in many everyday situations, they overlap a lot.

So both are good Spanish:

• Nos faltan dos sillas para la terraza
• Nos hacen falta dos sillas para la terraza

Usually it means we do not have enough.

So the most natural interpretation is:

• We are short by two chairs
• We still need two more chairs

It does not usually mean that two specific chairs have disappeared. If you wanted to say that two chairs are missing in the sense of gone, you would normally need more context.

In other words, here faltar is about shortage, not necessarily loss.

Yes, Spanish word order is flexible, but the original order is the most neutral and natural.

Most neutral:

• Nos faltan dos sillas para la terraza

You could also hear:

• Dos sillas nos faltan para la terraza

But that sounds more marked or emphatic, as if you are stressing two chairs in particular.

So for a learner, the safest default is:

• Nos faltan dos sillas para la terraza

Yes. It sounds natural, especially in a practical situation like preparing a meal, a gathering, or outdoor seating.

For example, if you are setting up for guests, someone might say:

• Nos faltan dos sillas para la terraza

It is a normal, idiomatic way to say that the current number of chairs is not enough. It sounds natural in Spain and elsewhere in the Spanish-speaking world.