Одинадцять людей у кімнаті.

Because after 5 and higher numbers in Ukrainian, the noun usually goes in the genitive plural.

So:

• один чоловік = one person
• два / три / чотири людини = two / three / four people
• п’ять людей = five people
• одинадцять людей = eleven people

Here, людей is the genitive plural form meaning of people.
The basic plural form люди would not be used after одинадцять.

Кімнаті is in the locative case.

That is because the preposition у here means in, showing a location. After у / в when you mean where something is, Ukrainian often uses the locative:

• у кімнаті = in the room
• у школі = in the school
• у місті = in the city

The dictionary form is кімната, but after у it becomes у кімнаті.

Ukrainian often omits the present-tense verb to be in simple statements.

So a sentence like this can work without any word for are:

• Одинадцять людей у кімнаті.
  literally: Eleven people in the room.

A fuller version is also possible:

• У кімнаті є одинадцять людей. = There are eleven people in the room.

So the short version is normal and natural, especially in simple descriptions.

Both у and в can mean in.

Ukrainian often chooses between them for sound and ease of pronunciation. This is called euphony. Speakers pick the form that sounds smoother in context.

So:

• у кімнаті
• в кімнаті

Both are possible in many situations. In this sentence, у кімнаті sounds natural and easy to say.

It is grammatical, but many speakers would more naturally say:

• У кімнаті одинадцять людей.

That order is very common when introducing what is in a place.

Compare:

• У кімнаті одинадцять людей. = In the room, there are eleven people.
• Одинадцять людей у кімнаті. = Eleven people are in the room.

The second version can sound a bit more like you are focusing on the eleven people themselves, while the first sounds more like a neutral statement about the room.

Одинадцять is one of the numerals from 5 upward, and these usually require the noun in the genitive plural.

Examples:

• одинадцять книг = eleven books
• одинадцять студентів = eleven students
• одинадцять людей = eleven people

So this sentence follows a very important Ukrainian pattern:

number 5+ + genitive plural noun

Yes. These forms belong to the same word family:

• людина = a person
• люди = people
• людей = of people / people after numbers like five, eleven, etc.

For learners, it is best to remember:

• singular: людина
• plural: люди
• genitive plural: людей

This word is a little irregular, so it is worth memorizing as a set.

The main stresses are:

• одина́дцять
• люде́й
• кімна́ті

So you can say it roughly like:

odynAdtsyat lyudEy u kimnAti

A more careful approximation for an English speaker:

• Одина́дцять ≈ oh-dih-NAHD-tsyat’
• люде́й ≈ lyoo-DEY
• у кімна́ті ≈ oo kim-NA-ti

The stress matters in Ukrainian, so it helps to learn each word together with its stressed syllable.