pureininzuu ni yotte, settoappu ga kawaru.

Here, によって means depending on or according to.

So:

• プレイ人数によって = depending on the number of players

This is a very common pattern:

• A によって B が変わる = B changes depending on A

In this sentence, the thing that causes variation is プレイ人数 and the thing that changes is セットアップ.

によって is used to show that something varies based on some condition or factor.

So:

• プレイ人数によって、セットアップが変わる。 = The setup changes depending on the number of players.

If you used other particles, the nuance would change or sound unnatural here.

For example:

• で often marks a means, place, or cause
• と often marks a partner, quote, or condition

But によって is especially natural when talking about differences that arise from a factor:

• 人によって考え方が違う。 = Ways of thinking differ depending on the person.

プレイ人数 means number of players.

It is made of:

• プレイ = play
• 人数 = number of people

So literally it is something like play-person-count, but naturally in English it means the number of players.

You might also sometimes see:

• プレイヤー人数
• 参加人数 depending on context

But in games, プレイ人数 is very common.

Because セットアップ is a borrowed word from English: setup.

Katakana is commonly used for:

• loanwords
• foreign names
• some technical terms
• emphasis

In game rules, words like these are very often written in katakana:

• セットアップ = setup
• プレイヤー = player
• カード = card
• ゲーム = game

Because セットアップ is the thing that changes.

The verb 変わる is an intransitive verb, meaning to change or to be different / become different. The thing undergoing the change is marked with が.

So:

• セットアップが変わる = The setup changes

Compare:

• 何が変わる？ = What changes?
• セットアップが変わる。 = The setup changes.

This is a very important pair.

• 変わる = to change / to be changed
  
  • intransitive
  • something changes on its own, or the sentence focuses on the result
• 変える = to change something
  • transitive
  • someone actively changes it

In this sentence:

• セットアップが変わる = The setup changes

If you said:

• セットアップを変える = change the setup

that would mean someone is actively changing it.

So the original sentence is not saying someone changes the setup, but rather the setup differs depending on player count.

Japanese often leaves out information that is clear from context.

In English, you might say:

• The setup changes depending on the number of players.
• In this game, the setup changes depending on player count.

In Japanese, if the context is already clear—such as in a rulebook—there is no need to mention the game, it, or you.

That makes the sentence more natural and efficient.

Because the sentence is just presenting a straightforward fact:

• プレイ人数によって、セットアップが変わる。

There is no strong need to mark a separate topic with は.

If you added は, the nuance could shift. For example:

• セットアップは、プレイ人数によって変わる。

This is also correct, and it puts more focus on the setup as the topic:
As for the setup, it changes depending on the number of players.

So the original sentence is simply a neutral rulebook-style statement.

Yes. You can think of it as a very common structure:

• [condition/factor] によって、[thing] が 変わる

Examples:

• 季節によって服装が変わる。 = Clothing changes depending on the season.
• 人によって意見が違う。 = Opinions differ depending on the person.
• 場合によって結果が変わる。 = The result changes depending on the situation.

So this sentence fits a very useful grammar pattern.

Because Japanese normally puts the main verb at the end of the sentence.

So the structure is roughly:

• プレイ人数によって = depending on the number of players
• セットアップが = the setup
• 変わる = changes

This is standard Japanese word order.

It can cover all of those ideas depending on context.

In this sentence, natural English translations include:

• The setup changes depending on the number of players.
• The setup differs depending on the number of players.
• The setup varies with the number of players.

Japanese 変わる is often broader than any single English word, so the best translation depends on the situation.

The comma after によって helps separate the condition from the main statement:

• プレイ人数によって、セットアップが変わる。

It makes the sentence easier to read, especially in written instructions or rules.

Japanese commas are often used more flexibly than English commas. In this sentence, it is helpful but not absolutely required. You may also see:

• プレイ人数によってセットアップが変わる。

with no comma.

Yes. A few natural alternatives are:

• プレイ人数でセットアップが変わる。
  Slightly less formal; still understandable as the setup changes depending on the number of players.

• プレイ人数によってはセットアップが変わる。
  This changes the meaning. It suggests for some player counts, the setup changes or depending on the number of players, it may change, so it is not the same as the original.

• セットアップはプレイ人数によって変わる。
  Same basic meaning, but with セットアップ as the topic.

The original sentence sounds very natural for a manual or rulebook.