A classe tem vinte alunos.

A means the.

It is the feminine singular definite article in Portuguese, and it matches classe, which is a feminine singular noun.

So:

• a classe = the class

Because classe is a feminine noun in Portuguese.

Portuguese nouns have grammatical gender, so the article must agree with the noun:

• o = masculine singular
• a = feminine singular

Since classe is feminine, you say a classe.

Tem means has in this sentence.

It is the third-person singular form of the verb ter (to have):

• eu tenho = I have
• você/ele/ela tem = you/he/she has

Here, the subject is a classe, which is singular, so Portuguese uses tem.

Because the verb agrees with the subject, not with the number of students.

The subject is a classe, and classe is singular. So the verb must also be singular:

• A classe tem... = The class has...

Even though vinte alunos is plural, it is not the subject.

Because vinte does not change for gender or number.

Most Portuguese numbers are invariable:

• vinte alunos = twenty male students / students in general
• vinte alunas = twenty female students

The number vinte stays the same.

Alunos is the masculine plural form of aluno.

In Portuguese, the masculine plural is often used for:

• a group of only males, or
• a mixed group of males and females

So vinte alunos can mean:

• twenty male students, or
• twenty students in general

If the group were all female, you would say vinte alunas.

In this sentence, classe means the class as a group of students.

Depending on context, Portuguese speakers may also use other words:

• turma = class/group of students
• sala de aula = classroom
• classe = class, often understandable but sometimes a bit more formal or context-dependent

So this sentence is natural, but in everyday Brazilian Portuguese, turma is also very common.

Yes. A turma tem vinte alunos is very natural in Brazilian Portuguese.

Both sentences mean basically the same thing:

• A classe tem vinte alunos.
• A turma tem vinte alunos.

In many everyday situations in Brazil, turma may sound more common than classe when talking about a group of students.

Because Portuguese normally does not use an article after a number in this kind of sentence.

So you say:

• vinte alunos = twenty students

Not:

• vinte os alunos

The number already introduces the noun phrase, so no article is needed there.

In Brazilian Portuguese, tem is pronounced with a nasal sound at the end.

It sounds roughly like teyn with a nasal ending, but not exactly like any English word.

A simple learner-friendly approximation is:

• tem ≈ teng / tein with the air coming through the nose

The final m is usually not pronounced like a full English m. It mainly nasalizes the vowel.

A rough Brazilian Portuguese pronunciation is:

• vinte ≈ VEEN-chee or VEEN-tee, depending on accent
• alunos ≈ ah-LOO-noos

Together:

• vinte alunos ≈ VEEN-chee ah-LOO-noos

In natural speech, the words may connect smoothly, almost like:

• vintea-lunos

Yes, in colloquial Brazilian Portuguese, tem is often used to mean there is or there are:

• Tem um livro na mesa. = There is a book on the table.
• Tem muitos alunos aqui. = There are many students here.

But in A classe tem vinte alunos, tem clearly means has, because a classe is the subject.

Yes. It is a normal and straightforward sentence in Portuguese.

Portuguese often expresses this idea with to have, just like English can:

• The class has twenty students.

Another common way would be:

• Há vinte alunos na classe. = There are twenty students in the class.

But A classe tem vinte alunos is perfectly natural.