kantanna tetuduki ni site ha, hituyouna syorui ga ooi.

にしては means something like:

• for...
• considering...
• for something that is supposed to be...

It shows that the speaker thinks the result is surprising or does not match what would normally be expected.

So in 簡単な手続きにしては、必要な書類が多い, the idea is:

• If it is a simple procedure, you would expect fewer required documents.
• But actually there are a lot.
• So the second part feels unexpected.

A natural English gloss is:

• For a simple procedure, there are a lot of required documents.

Yes. Historically and grammatically, にしては comes from にする / にして plus は.

In modern Japanese, though, learners usually treat 〜にしては as a set expression meaning:

• for
• considering
• in spite of being

So while it is connected to する, in this kind of sentence it is best learned as one grammar pattern: X にしては Y.

Pattern:

• Noun + にしては
• Plain form + にしては in some cases as well

Here, 簡単な手続き is treated as the thing being evaluated, and にしては means for a simple procedure.

Because 簡単 is a na-adjective.

When a na-adjective directly modifies a noun, it usually takes な:

• 簡単な手続き = a simple procedure
• 静かな部屋 = a quiet room
• 便利なアプリ = a convenient app

So:

• 簡単 by itself can mean simple
• but before 手続き, you need 簡単な

That is why the first part is 簡単な手続き.

手続き means procedure, process, or formal steps.

Depending on context, it can refer to things like:

• an application procedure
• administrative paperwork
• official processing steps
• some formal process you need to complete

In this sentence, it probably means something like an official or practical procedure, especially because 必要な書類 follows it. That combination strongly suggests paperwork or administration.

Here, が marks 必要な書類 as the thing that is being described by 多い.

Structure:

• 必要な書類 = required documents
• が = subject marker
• 多い = are many / are numerous

So the sentence is saying:

• The required documents are many.

Using が is very natural when identifying what is many, few, expensive, difficult, etc.

If you used は, it would create a stronger contrast, something like:

• As for the required documents, they are many which is possible in some contexts, but less neutral here.

So が is the normal choice.

Because 多い is being used as the main predicate of the sentence.

Compare:

1. 必要な書類が多い

   • The required documents are many
   • Here 多い finishes the sentence.

2. 多くの必要な書類

   • many required documents
   • Here 多くの modifies a noun that comes after it.

In this sentence, nothing comes after 多い, so it is acting as the predicate:

• 必要な書類が多い = There are many required documents.

That is why 多い is correct here.

They are similar, but not exactly the same.

• 〜にしては = for... / considering...; compares reality to what would normally be expected
• 〜のに = although / even though; more directly expresses contradiction

So:

• 簡単な手続きにしては、必要な書類が多い = For a simple procedure, there are a lot of required documents. = emphasizes judgment based on expectation

Compare:

• 簡単な手続きなのに、必要な書類が多い = Even though it’s a simple procedure, there are a lot of required documents. = stronger direct contradiction

Both can work, but にしては is especially good when you mean:

• given that it is called/simple as a category, this is more than expected

The nuance is mildly evaluative or slightly critical.

The speaker is basically saying:

• This procedure is supposed to be simple.
• For something simple, I would expect less paperwork.
• But in reality, the number of required documents is surprisingly high.

So it often carries a feeling like:

• That’s more than I expected
• That seems excessive
• It’s not as simple as it sounds

It is not just a neutral fact; it includes the speaker’s sense of mismatch between expectation and reality.

The comma shows a pause after the comparison phrase:

• 簡単な手続きにしては、
• 必要な書類が多い。

This is very common in Japanese writing when the first part sets up the condition, frame, or contrast.

The comma is not absolutely required in every case, but it is natural and helps readability. It separates:

• the standard of comparison: for a simple procedure
• the main comment: there are many required documents

Yes, Japanese often leaves out things that are understood from context.

A fuller version in context might be something like:

• この手続きは、簡単な手続きにしては、必要な書類が多い。

But Japanese does not need to state this procedure if the topic is already obvious.

So the sentence can naturally stand on its own, with the meaning understood as something like:

• For a simple procedure, it has a lot of required documents.

Omitting the obvious topic is very normal in Japanese.

Yes, that is a very good way to understand it.

にしては often has the nuance of:

• for something of that type
• considering that it is classified as X
• for something said to be X

So 簡単な手続きにしては is not just describing the procedure as simple. It also implies:

• if we call this a simple procedure,
• or if we judge it by the standards of a simple procedure,

then the number of required documents is surprisingly large.

That is why にしては feels more nuanced than a simple because it is simple type of expression.