عدد الناس مهم.

A careful MSA pronunciation is:

ʿadadu n-nāsi muhimmun

With full case endings and vowels, it is written:

عَدَدُ النَّاسِ مُهِمٌّ

A few notes:

• عدد = ʿadad
• الناس = an-nās, but because ل assimilates to the ن, it is pronounced an-nās / n-nās
• مهم = muhimm

In pause, many speakers would stop with:

ʿadad an-nās muhimm

Because Arabic often uses nominal sentences in the present tense without an explicit verb meaning to be.

So in the present, Arabic simply says:

• عدد الناس مهم
• literally: the number of people important

This naturally means The number of people is important.

If you wanted past or future, then Arabic would use other forms, such as a verb like كان for was.

This is a nominal sentence (جملة اسمية).

It has two main parts:

• عدد الناس — the subject/topic (مبتدأ)
• مهم — the predicate/comment (خبر)

So the structure is:

• [subject] + [predicate]
• عدد الناس + مهم

This is one of the most basic and common sentence patterns in Arabic.

Because Arabic often expresses X of Y using a structure called the iḍāfa (the genitive construction).

So:

• عدد الناس = the number of the people / the number of people

In English, you need of.
In Arabic, you usually just put the two nouns next to each other:

• first noun: عدد = number
• second noun: الناس = the people

Together they form an iḍāfa phrase.

In an iḍāfa construction, the order is fixed:

• first comes the thing being possessed or specified
• then comes what specifies it

So Arabic says:

• عدد الناس = number of people

not:

• الناس عدد for this meaning

The first noun (عدد) is the head noun, and the second noun (الناس) tells you whose number or the number of what.

Because الناس itself means the people and is a definite noun.

Also, in an iḍāfa, the definiteness of the whole phrase depends on the second noun. Since الناس is definite, عدد الناس becomes definite too.

So:

• عددُ ناسٍ = a number of people
• عددُ الناسِ = the number of the people / the number of people

That is why this phrase can function naturally as the number of people.

Because مهم agrees with عدد, not with الناس.

The subject here is the whole phrase عدد الناس, but its head noun is عدد:

• عدد = singular
• عدد = masculine

So the predicate is:

• مهم = masculine singular

Arabic is treating the sentence as being about the number, not directly about the people.

In an iḍāfa construction, the first noun normally does not take ال.

So Arabic says:

• عدد الناس

not:

• العدد الناس

The first noun becomes definite through the second noun. Since الناس is definite, the whole phrase is definite already.

This is a very important rule of iḍāfa.

In sentences like this, the predicate often remains indefinite.

So:

• عدد الناس = definite subject
• مهم = indefinite predicate

This is normal Arabic sentence structure. The predicate is describing the subject, not necessarily matching it in definiteness.

So Arabic prefers:

• عدد الناس مهم

rather than:

• عدد الناس المهم

because المهم would usually sound like the important one/thing, not a simple predicate in this sentence.

With full case endings, the sentence is:

عَدَدُ النَّاسِ مُهِمٌّ

Here is why:

• عَدَدُ → -u because it is the subject (mubtadaʾ), so nominative
• النَّاسِ → -i because it is the second term of an iḍāfa, so genitive
• مُهِمٌّ → -un because it is the predicate (khabar) and nominative

In normal unvowelled writing, these endings are usually not written, so learners just see:

عدد الناس مهم

Yes, but عدد الناس مهم is the most neutral and straightforward order.

This order is best for learners because it clearly gives:

• subject first
• predicate second

You may sometimes see stylistic variations in Arabic, especially in formal writing, but this basic order is the standard one to learn first.

So if your goal is clear MSA, عدد الناس مهم is exactly the right pattern.

In meaning, الناس refers to people, so it is plural in sense.

However, in this sentence, that does not control the agreement, because the main noun is عدد.

That is why you should focus on:

• عدد = the grammatical head of the subject phrase
• الناس = the noun inside the iḍāfa

So even though الناس means many people, the predicate still appears as singular masculine: مهم.

You could do that as a rough grammar aid, but not as good English.

A more useful learner breakdown is:

• عدد = number
• الناس = the people / people
• مهم = important

And the natural English sentence is:

• The number of people is important

So word-for-word breakdowns can help you see the structure, but they should not replace the natural translation.