Seize personnes sont à la brocante.

A natural pronunciation is:

/sɛz pɛʁ.sɔn sɔ̃.t‿a la bʁɔ.kɑ̃t/

A rough English-friendly guide is:

sez pair-sonn son-ta la bro-kahnt

A few helpful notes:

• seize sounds like sez
• personnes has 2 syllables: per-sonne
• sont à has a liaison, so the silent t in sont is pronounced before à
• the r in French is not the same as in English
• -an- / -en- / -am- / -em- type sounds in French are often nasal; in brocante, the -can- part is nasal

Because the subject is plural: seize personnes = sixteen people.

French être changes depending on the subject:

• je suis = I am
• tu es = you are
• il/elle est = he/she is
• nous sommes = we are
• vous êtes = you are
• ils/elles sont = they are

Since sixteen people is plural, French uses the plural form sont.

In French, when you give a number directly before a noun, you usually do number + noun with no article:

• seize personnes
• trois voitures
• deux enfants

So seize personnes is normal.

You would only add an article in certain more specific situations, for example:

• les seize personnes = the sixteen people
  when you mean a specific group already known from context

Because the number is more than one, so the noun is plural.

• une personne = one person
• deux personnes = two people
• seize personnes = sixteen people

This is very similar to English, where person becomes people or persons depending on style. In ordinary French, you simply make personne plural as personnes.

Personnes is the natural choice after a specific number.

• seize personnes = normal
• seize gens = not natural in standard everyday French

French often uses gens for people in a general sense:

• Les gens sont gentils ici. = People are nice here.

But after exact numbers, French usually prefers personnes:

• Il y a vingt personnes. = There are twenty people.

So in this sentence, personnes is exactly what you would expect.

Here, à la brocante means at the flea market, at the second-hand market, or at the antiques/used-goods fair, depending on context.

A brocante is a place or event where old or second-hand items are sold. It is not exactly the same as a regular shop.

So the phrase suggests that the sixteen people are physically present there.

French often uses à where English uses at.

So:

• être à l'école = to be at school
• être au marché = to be at the market
• être à la brocante = to be at the flea market

Using dans would focus more on being inside something physically. With an event or market, à is the normal choice.

So sont à la brocante means are at the flea market, not movement toward it.

Because brocante is a feminine noun in French.

So:

• la brocante = the flea market
• not le brocante

This is something you generally have to learn with the noun. In French, nouns have grammatical gender, and the article must match:

• le for masculine
• la for feminine
• l' before a vowel sound

Yes. Personne can be:

1. a noun meaning person
2. an indefinite pronoun meaning nobody / no one

Examples:

• une personne = a person
• Il n'y a personne. = There is nobody.

In Seize personnes sont à la brocante, it clearly means people/persons because:

• it comes after a number: seize
• it is plural: personnes

So there is no ambiguity here.

No, the word order is quite straightforward.

French structure here is:

Seize personnes + sont + à la brocante
subject + verb + place

That matches English fairly closely:

Sixteen people + are + at the flea market

So this is a very standard sentence pattern in French.

Yes: sont à normally has a liaison.

So instead of saying the two words separately, French usually links them:

• sont à → /sɔ̃.t‿a/

That means the normally silent final t in sont is heard before à.

But there is no liaison between seize and personnes, because personnes begins with p, a consonant sound.