kono syatu ha botan ga ooi node, kiruno ni sukosi zikan ga kakarimasu.

Because the sentence has a topic and then a description about that topic.

• このシャツは = as for this shirt
• ボタンが多い = the buttons are many / it has a lot of buttons

So the whole idea is:

• As for this shirt, the buttons are many, so...

Here, このシャツ is the main topic of the sentence, so it takes は.

Inside that topic, ボタン is the thing being described by 多い. With 多い, the thing that is numerous is commonly marked by が.

So ボタンが多い is the natural pattern.

ので means because / since and gives the reason for what follows.

In this sentence:

• ボタンが多いので = because it has a lot of buttons

Compared with から, ので often sounds a little more neutral, softer, or more explanatory.

• ボタンが多いから = because it has a lot of buttons
• ボタンが多いので = because it has a lot of buttons

Both are correct, but ので can feel a bit more polite or objective.

This is a very common grammar pattern:

• dictionary form verb + のに + 時間がかかる

It means:

• it takes time to do...

So:

• 着る = to put on / to wear
• 着るの = the act of putting on
• 着るのに時間がかかる = it takes time to put on

Here, の turns the verb 着る into a noun-like phrase, so the action itself can be talked about as a thing.

Then に connects that action to 時間がかかる.

So this part means:

• it takes a little time to put it on

No. It looks the same, but here it is a different structure.

In this sentence:

• 着るのに = for putting on / to put on

This is:

• 着る + の + に

It is part of the pattern:

• Verb + のに + 時間がかかる
• Verb + のに + お金がいる
• Verb + のに + 便利だ

That is different from the conjunction のに meaning although / even though.

For example:

• 雨なのに、出かけた。
• Even though it was raining, I went out.

So the のに in your sentence does not mean although.

These are related, but they are used differently.

• 時間がかかる = time is required / it takes time
• 時間をかける = to spend time on something

In your sentence, the speaker is describing the action in a general way:

• 着るのに少し時間がかかります
• It takes a little time to put on

This focuses on the fact that the action requires time.

If you used 時間をかける, it would focus more on the person intentionally spending time.

So:

• この作業は時間がかかる。 = This task takes time.
• 私はこの作業に時間をかける。 = I spend time on this task.

Because 少し is modifying 時間.

• 少し時間 = a little time

So:

• 少し時間がかかります = it takes a little time

This word order is very natural in Japanese.

You may also see:

• 時間が少しかかります

That is also possible, but the nuance is slightly different in emphasis.
In your sentence, 少し時間がかかります presents a little time as a chunk.

多い means many / numerous, and it is commonly used when something exists in a large number.

So:

• ボタンが多い = there are a lot of buttons / it has many buttons

This is a natural way to describe a shirt with many buttons.

A similar expression would be:

• ボタンがたくさんある

That also means there are many buttons.
The version with 多い is compact and very common.

Here, 着る means to put on.

Japanese 着る can refer to both:

• put on
• wear

The exact meaning depends on context.

In this sentence, because it says it takes time, the meaning is clearly put on, not just wear in a general state.

Also, 着る is the normal verb for putting on upper-body clothing, such as:

• shirts
• jackets
• coats

So it fits シャツ perfectly.

Yes. There are several natural alternatives. For example:

• このシャツはボタンが多いので、着るのにちょっと時間がかかります。
• このシャツはボタンがたくさんあるので、着るのに少し時間がかかります。

These mean basically the same thing.

Differences:

• 少し sounds a bit more neutral or formal than ちょっと
• ボタンが多い is more compact than ボタンがたくさんある

But your original sentence is completely natural.