Het nummer is een miljoen.

Because nummer is a het-word in Dutch: het nummer.

In Dutch, singular nouns usually take either de or het as their definite article, and you mostly have to learn this with each noun.

• het nummer = the number
• not de nummer

So het here is simply the word for the that matches nummer.

Not exactly.

Both can relate to numbers, but they are used differently:

• nummer often means a number as a label or identifier
  
  • a phone number
  • a house number
  • a bus number
  • a song/track number
• getal means a number in the mathematical sense

So if you are talking about a mathematical value, many Dutch speakers would more naturally say:

• Het getal is een miljoen.

Your sentence Het nummer is een miljoen is still understandable, but it may sound more like a number assigned to something, depending on context.

Because is is the third-person singular form of zijn (to be).

The subject is het nummer, which is singular, so Dutch uses:

• ik ben = I am
• jij bent = you are
• hij/zij/het is = he/she/it is

So:

• Het nummer is ... = The number is ...

Because miljoen is a noun in Dutch, just like million is in English.

So Dutch uses an article before it:

• een miljoen = a million / one million

This works the same way in longer numbers too:

• twee miljoen = two million
• drie miljoen = three million

This is a very common question.

• een miljoen usually means a million or simply one million
• één miljoen is used when you want to stress the number one

So:

• Het nummer is een miljoen. = neutral
• Het nummer is één miljoen. = specifically emphasizing one million, not two million, not ten million, etc.

The accents in één are not always required in casual writing, but they are helpful when you want to make the meaning clear.

Because the sentence refers to one million.

Compare:

• een miljoen = one million
• twee miljoen = two million
• drie miljoen = three million

After another number, Dutch often keeps miljoen in the singular form. But when you talk about millions in a general sense, you can also see the plural noun:

• miljoenen mensen = millions of people

So in your sentence, een miljoen is singular because it means exactly one million.

Yes, in this simple sentence it is very similar to English:

• Het nummer = subject
• is = verb
• een miljoen = complement

So Dutch follows the same basic pattern here:

subject + verb + complement

That gives:

• Het nummer is een miljoen.

This is one reason beginner Dutch sentences can sometimes look very familiar to English speakers.

miljoen is pronounced roughly like mil-YOON.

A simple English-style approximation:

• mil as in mill
• joen sounding roughly like yoon

So:

• miljoen ≈ mil-YOON

And the whole sentence is approximately:

• het NUM-mer is un mil-YOON

A few notes:

• het is often pronounced quickly, almost like hut or a reduced ut
• nummer has stress on the first syllable: NUM-mer
• miljoen has stress on the last syllable: mil-JOEN

Yes. nummer can also mean things like:

• a song or track
• an item number
• an act in a performance
• a sports player’s number

For example:

• een mooi nummer = a nice song
• mijn nummer is 12 = my number is 12

So Dutch learners often need to rely on context. In your sentence, if the meaning has already been shown, that removes the ambiguity.

It depends on the context.

If you mean a numerical identifier, it can work:

• a ticket number
• a house number
• a contestant number

But if you mean a number as a mathematical value, many speakers would prefer:

• Het getal is een miljoen.

So the sentence is grammatically fine, but nummer may or may not be the most natural noun depending on what kind of number you mean.

No. In normal Dutch, miljoen is not capitalized.

So the standard form is:

• Het nummer is een miljoen.

Dutch capitalization rules are broadly similar to English for this kind of sentence: you capitalize the first word of the sentence, but not ordinary nouns like nummer or miljoen.