Negentig mensen zijn aanwezig.

Because the subject is plural: negentig mensen.

In Dutch, the verb zijn changes like this in the present tense:

• ik ben = I am
• jij/u bent = you are
• hij/zij/het is = he/she/it is
• wij/jullie/zij zijn = we/you/they are

Since mensen is plural, Dutch uses zijn:

• Negentig mensen zijn aanwezig.

Not:

• Negentig mensen is aanwezig.

The singular is mens, and the plural is mensen.

This is an irregular plural pattern, so it is something you simply need to learn:

• de mens = the person / the human being
• de mensen = the people

Also, after numbers greater than one, Dutch normally uses the plural noun:

• één mens = one person
• twee mensen = two people
• negentig mensen = ninety people

Because Dutch usually does not use an article before a number + noun combination when you are speaking generally about a quantity.

So Dutch says:

• negentig mensen = ninety people
• drie boeken = three books
• tien huizen = ten houses

You would not normally say:

• de negentig mensen unless you mean the ninety people specifically
• een negentig mensen is not correct here

So in your sentence, negentig mensen just means a quantity, not a specific group already identified by the.

Because aanwezig is being used predicatively, not directly before a noun.

Compare these two patterns:

• de aanwezige mensen = the people who are present
• de mensen zijn aanwezig = the people are present

When an adjective comes before a noun, it often takes -e:

• een aanwezige gast
• de aanwezige gasten

But after a linking verb like zijn, the adjective usually stays in its basic form:

• De gasten zijn aanwezig.
• Negentig mensen zijn aanwezig.

So aanwezig has no -e here because it is part of the predicate.

It is an adjective.

In this sentence, it works as a predicate adjective after zijn, just like in English:

• The room is quiet.
• De kamer is stil.

And here:

• Negentig mensen zijn aanwezig.

So aanwezig describes the state of the people: they are present.

You can also see it before a noun, where it is clearly an adjective:

• de aanwezige mensen = the people present / the people who are there

This is normal Dutch word order.

In a main clause, the finite verb usually comes in the second position. Here the subject comes first:

• Negentig mensen = subject
• zijn = finite verb
• aanwezig = complement

So the structure is:

• Negentig mensen | zijn | aanwezig.

Dutch often places the rest of the information after the finite verb, and predicate adjectives like aanwezig commonly come later in the clause.

Yes, absolutely. That is a very common alternative.

Both are correct:

• Negentig mensen zijn aanwezig.
• Er zijn negentig mensen aanwezig.

The difference is mainly one of emphasis:

• Er zijn negentig mensen aanwezig sounds more like There are ninety people present. It introduces the existence/number of people.
• Negentig mensen zijn aanwezig puts negentig mensen first, so it highlights that group or number more directly.

In many everyday situations, Er zijn negentig mensen aanwezig may sound slightly more natural.

You can use both, but they are not identical in tone.

• mensen = normal, everyday word for people
• personen = more formal, official, or administrative

So:

• Negentig mensen zijn aanwezig. = normal and natural
• Negentig personen zijn aanwezig. = more formal

You might see personen in official writing, reports, signs, or legal contexts.

It sounds fairly neutral, but a little formal because of aanwezig.

In many everyday situations, Dutch speakers might also say things like:

• Er zijn negentig mensen.
• Er zijn negentig mensen hier.
• Negentig mensen zijn er.

But aanwezig is very common in:

• attendance lists
• meetings
• presentations
• announcements
• written reports

So the sentence is perfectly normal, especially in a formal or semi-formal context.

A learner usually notices two things:

• the g sound is not like English g in go
• the middle of the word is often said quite quickly in normal speech

Roughly, negentig is pronounced something like:

• NAY-ghen-tikh or NAY-ghen-tuhkh

A few pronunciation notes:

• ne- sounds like nay
• the g is a Dutch throat sound, not an English hard g
• the final -ig is often pronounced with a soft, reduced vowel plus a rough consonant sound, depending on accent

You do not need perfect pronunciation right away, but it helps to know that Dutch g and final -ig do not sound the way an English speaker might expect from spelling alone.