Zestig mensen wachten buiten.

Because the subject is plural: zestig mensen = sixty people.

In the present tense, Dutch uses:

• ik wacht = I wait / am waiting
• jij wacht = you wait
• hij/zij wacht = he/she waits
• wij/jullie/zij wachten = we/you/they wait

Since mensen is plural, the verb takes the plural form wachten.

Because mens has an irregular plural.

• singular: de mens = the human / person
• plural: de mensen = the people / persons

This is a very common word, so it is worth memorizing as a set:

• een mens
• twee mensen

In Dutch, just like in English, a number can go directly before a plural noun:

• zestig mensen = sixty people
• drie boeken = three books
• tien huizen = ten houses

You would only add an article if you mean a specific group:

• de zestig mensen wachten buiten = the sixty people are waiting outside

So without de, the sentence simply states that sixty people are waiting outside.

This follows the normal Dutch word order for a main clause:

subject + finite verb + other information

So here:

• Zestig mensen = subject
• wachten = finite verb
• buiten = adverb of place

Dutch main clauses usually place the finite verb in the second position. Since the subject comes first here, the verb comes right after it.

Buiten is an adverb of place, meaning outside or outdoors. In a simple Dutch sentence, adverbs of place often come later in the sentence, often near the end.

So:

• Zestig mensen wachten buiten.

This is very natural Dutch.

You can move buiten for emphasis, but then the word order changes:

• Buiten wachten zestig mensen. = Outside, sixty people are waiting.

That sounds more marked or emphatic.

It can mean both, depending on context.

Dutch present tense often covers both:

• wait
• am/is/are waiting

So Zestig mensen wachten buiten can be understood as:

• Sixty people wait outside
• Sixty people are waiting outside

In normal English, are waiting is usually the more natural translation in this context.

Because wachten does not always need op.

• wachten by itself = to wait / to be waiting
• wachten op = to wait for

In your sentence, the idea is simply that the people are waiting outside. It does not say what they are waiting for.

Compare:

• Zestig mensen wachten buiten. = Sixty people are waiting outside.
• Zestig mensen wachten buiten op de bus. = Sixty people are waiting outside for the bus.

Yes. That is also correct, and often very natural.

• Zestig mensen wachten buiten.
• Er wachten zestig mensen buiten.

The version with er is similar to English there are in some contexts. It introduces the existence of people:

• Er wachten zestig mensen buiten. = There are sixty people waiting outside.

This version is especially common when introducing new information.

Usually yes, but it can also mean outdoors, depending on context.

In this sentence, buiten means that the people are not inside; they are outside the building or outside somewhere nearby.

So possible English renderings are:

• outside
• outdoors

Usually outside is the most direct translation here.

A simple learner-friendly approximation is:

• Zestig → ZES-tikh
• mensen → MEN-sen
• wachten → WAKH-ten
• buiten → BOW-ten or BOW-tuh(n)

A few pronunciation notes:

• Dutch z sounds like English z
• ch in wachten is the rough Dutch throat sound
• ui in buiten is a typical Dutch vowel sound that English does not have exactly, so BOW-ten is only an approximation

Yes, unless it starts the sentence.

Numbers written out as words are normally lowercase in Dutch:

• zestig mensen
• twintig boeken

In your sentence, Zestig is capitalized only because it is the first word of the sentence.

Yes, grammatically that works, but mensen is much more natural in everyday Dutch.

• mensen = people
• personen = persons

Personen sounds more formal, official, or administrative. In normal speech, Dutch speakers usually prefer mensen.

So:

• Zestig mensen wachten buiten. = normal everyday Dutch
• Zestig personen wachten buiten. = more formal or official tone