Nove pessoas estão aqui.

Because estar is used for location.

• Nove pessoas estão aqui = nine people are here / are located here.
• ser would usually identify or define something, not say where it is.

So:

• As pessoas estão aqui = the people are here
• As pessoas são brasileiras = the people are Brazilian

Using são aqui would sound wrong in this sentence.

Estão is the 3rd person plural form of estar in the present tense.

The subject is nove pessoas, which is plural, so the verb must also be plural.

Present tense of estar:

• eu estou
• você/ele/ela está
• nós estamos
• vocês/eles/elas estão

Since nove pessoas = they, you use estão.

Because nove means more than one, so the noun must be plural.

• uma pessoa = one person
• duas pessoas
• nove pessoas

This works like English: one person, but nine people.

In Portuguese, when you simply say a number plus a noun, you normally do not use an article.

So:

• nove pessoas
• três carros
• duas casas

That is the normal structure for counting.

You would only add an article in more specific situations, such as:

• As nove pessoas estão aqui = The nine people are here

That version refers to a specific group already known in the conversation.

Yes, pessoa is a feminine noun:

• a pessoa
• as pessoas

In this sentence, gender does not change the verb estão, because Portuguese verbs do not change for gender.

But gender would matter with articles and adjectives:

• duas pessoas brasileiras
• essas pessoas estão aqui

So in Nove pessoas estão aqui, the important agreement is mainly plural agreement with the verb.

Yes, but the meaning and focus are a little different.

• Nove pessoas estão aqui = nine people are here
  
  • Focus on those people and their location.
• Há nove pessoas aqui = there are nine people here
  
  • More formal, neutral, existence-focused.
• Tem nove pessoas aqui = there are nine people here
  
  • Very common in spoken Brazilian Portuguese.

So all three can be natural, but they are not identical in structure.

Yes, aqui can move, but the original order is very natural:

• Nove pessoas estão aqui.

You may also hear:

• Aqui estão nove pessoas.

This version is possible, but it sounds more marked or more like you are presenting what is in a place.

In everyday speech, Nove pessoas estão aqui is straightforward and easy to understand.

Yes, aqui usually means here.

It refers to a place near the speaker.

Related words:

• aqui = here
• ali = there, over there
• lá = there, farther away

In Brazilian Portuguese, aqui is extremely common and natural.

Sometimes, but aqui is much more common and neutral in Brazilian Portuguese.

• aqui = standard everyday here
• cá can also mean here, but it is less common in many Brazilian contexts and often appears in certain expressions.

So for a learner, aqui is the safest choice.

A rough pronunciation is es-TOWN, but with a nasal ending.

Important points:

• Stress is on the last syllable: -tão
• The ão sound is nasal, a very common Portuguese sound
• The s in estão sounds like s in most Brazilian pronunciations here

A rough guide:

• estão ≈ es-TOW̃

The tilde ~ over ã shows nasalization.

A rough Brazilian pronunciation is:

peh-SOH-as

Notes:

• Stress is on so
• The o in the stressed syllable is open in many accents
• The final s may sound like s in many Brazilian accents

So a rough learner-friendly version is:

• pessoas ≈ peh-SOH-as

No. Portuguese usually does not need a subject pronoun when the subject is already stated.

Since you already have Nove pessoas as the subject, adding elas would be unnecessary.

Natural:

• Nove pessoas estão aqui.

Less natural in this context:

• Nove pessoas, elas estão aqui.

Portuguese often leaves pronouns out unless they are needed for emphasis or contrast.

Yes. Pessoas means people/persons in a general human sense and is not limited to women, even though the noun itself is grammatically feminine.

So nove pessoas can refer to:

• nine women
• nine men
• a mixed group

Grammatical gender and real-world gender are not always the same thing.

Yes, it is grammatical and natural.

It works especially well when you are talking about a specific group and saying where they are.

Depending on context, Brazilians might also say:

• Tem nove pessoas aqui.
• Há nove pessoas aqui.

But Nove pessoas estão aqui is absolutely correct and natural.