jīntiān gōngyuán lǐ rén hěn shǎo.

A helpful way to see it is:

今天 = today
公园里 = in the park
人 = people
很少 = few / not many

So the structure is basically:

time + place/topic + noun + adjective/comment

More naturally:

As for today, in the park, people are few.

This is a very common Mandarin pattern. Chinese often sets the scene first, then gives the comment.

今天 is a time word, and time words often come near the beginning of a Chinese sentence.

So 今天公园里人很少 sounds very natural:
Today, there are few people in the park.

You can also move time words in some cases, but putting them early is one of the most common patterns in Mandarin.

For example:

今天我很忙。 = Today I’m very busy.
明天他不来。 = Tomorrow he isn’t coming.

里 means inside / in.

So:

公园 = park
公园里 = in the park / inside the park

It works like a location word after a noun.

Other examples:

家里 = at home / in the house
学校里 = in the school
房间里 = in the room

So 公园里人很少 literally has the sense of inside the park, people are few.

Good question. You often can say 在公园里, but Chinese sometimes leaves out 在 when the location is being used as a topic or setting.

So both of these are possible:

今天公园里人很少。
今天在公园里，人很少。

The version without 在 is very natural and concise. It feels like:

As for in the park today, there are few people.

Chinese often allows this kind of location phrase without 在, especially in descriptive sentences.

Not always. In Chinese, 很 is often used before an adjective even when it does not strongly mean very.

Here, 少 is an adjective meaning few / little.
So 人很少 is the normal way to say there are few people.

If you just say 人少, it can sound contrastive or incomplete in some contexts, like:

People are few—as opposed to something else.

So in many basic descriptive sentences, 很 is used mainly to make the sentence sound natural.

That said, sometimes 很少 really can mean very few depending on context. Here it is often best understood simply as few or not many.

Because 少 is an adjective, not a noun.

In Mandarin, adjectives can directly act as the predicate of a sentence. You do not need 是 before them.

So:

人很少 = People are few
not 人是很少

Compare:

他很忙。 = He is busy.
天气很冷。 = The weather is cold.

But with nouns, 是 is common:

他是老师。 = He is a teacher.

So this sentence uses an adjective predicate, which is why 是 is not used.

Because 人 by itself already means people in a general sense.

人们 also means people, but it is used differently. It is more like people in a broad, collective, sometimes slightly formal sense. It would sound unnatural here.

So:

公园里人很少 = natural
公园里人们很少 = unnatural in this context

Also, 人们 is not usually used just to count or describe how many people are in a place.

In standard Mandarin, 今天公园里人很少 is the more natural choice.

Why? Because here 人 is the thing being talked about, and 很少 is the comment about it:

人很少 = people are few

很少人 can exist as a phrase meaning very few people, but in this sentence 人很少 is the normal and smoother structure.

So a learner should strongly prefer:

今天公园里人很少。

Yes, you can.

今天公园里人很少 and 今天公园里的人很少 are both understandable and natural.

The version without 的 is a little more direct and conversational.
The version with 的 turns 公园里的人 into a clearer noun phrase: the people in the park.

So:

今天公园里人很少。 = very natural, concise
今天公园里的人很少。 = also natural, slightly more explicit

Both can mean There are few people in the park today.

In English, we naturally say There are few people in the park today. Chinese does not always use a direct equivalent of there are.

Instead, Mandarin often uses a descriptive structure like:

公园里人很少
literally: In the park, people are few

So even though the English translation uses there are, the Chinese sentence does not need a separate word for there are here.

This is one reason Chinese sentence patterns can feel different from English: the meaning is the same, but the structure is often more like setting + thing + description.

Here, 少 is an adjective.

It describes the amount of 人:

人很少 = people are few / there are not many people

Chinese adjectives often behave a bit like English verbs such as to be busy, to be cold, to be few, because they can directly be the predicate of a sentence.

So in this sentence, 少 is best understood as an adjective meaning few / not many.