sankasya ga ooi you desu.

It breaks down like this:

• 参加者 = participant(s)
• が = subject marker
• 多い = many / numerous
• ようです = it seems / it appears

So the overall pattern is:

[noun] + が + [い-adjective] + ようです

Literally, it is something like:

As for the participants, they seem to be many.

More natural English is:

There seem to be many participants or It seems there are many participants.

Yes. 多い is an い-adjective.

In Japanese, 多い is commonly used to say that the number of something is large:

• 参加者が多い = There are many participants
• 質問が多い = There are many questions

This can feel a little different from English, because English often uses many directly before a noun, but Japanese often uses 多い as the main predicate of the sentence.

A useful point:

• 多い describes that something exists in large numbers
• it does not usually go directly before a noun as 多い参加者

Instead, if you want to say many participants before a noun, you usually say:

• 多くの参加者 = many participants

が marks 参加者 as the subject of the sentence: the thing being described as 多い.

So:

• 参加者が多い = The participants are many / There are many participants

Using は is also possible in some contexts:

• 参加者は多いです

But the nuance changes a little:

• が is more neutral and straightforward
• は makes 参加者 the topic, and can sound contrastive depending on context

For example:

• 参加者が多いです = There are many participants
• 参加者は多いです = As for the participants, they are many

If you are just making a plain observation, が is very natural here.

ようです means it seems, it appears, or it looks like.

It shows that the speaker is not stating the fact completely directly, but is making a judgment based on some kind of evidence or impression.

So:

• 参加者が多いです = There are many participants
• 参加者が多いようです = It seems there are many participants

This makes the sentence softer and less direct.

Not usually.

In this sentence, ようです normally suggests an inference or appearance, not simple hearsay.

That means the speaker is probably judging from evidence, such as:

• what they can see
• a report or data they checked
• the situation around them

If you specifically want I heard that there are many participants, Japanese more often uses そうです:

• 参加者が多いそうです = I heard there are many participants

So:

• ようです = it seems / it appears
• そうです = I heard / they say

The difference is mainly certainty and tone.

• 参加者が多いです = a direct statement
• 参加者が多いようです = a softer, less certain statement

Compare:

• 参加者が多いです。
  There are many participants.

• 参加者が多いようです。
  It seems there are many participants.

So ようです is useful when:

• you do not want to sound too absolute
• you are judging from evidence
• you want a more careful or polite tone

Sometimes, but the nuance changes.

There are two important そうです patterns in Japanese:

1. Hearsay そうです

   • 参加者が多いそうです
     = I heard there are many participants

2. Appearance そうです
   With adjectives, this would become:

   • 参加者が多そうです
     = It looks like there will be many participants / It seems numerous

Compared with ようです:

• 多そうです often sounds more like a surface impression or forecast
• 多いようです can sound a bit more reasoned or evidence-based
• ようです is often slightly more formal

So they are similar, but not interchangeable in every situation.

Because 多い itself works as the predicate.

Japanese does not always need a verb like to be or to exist where English does. An adjective can be the main part of the sentence.

So:

• 参加者が多い = Participants are many / There are many participants

That is already a complete sentence.

A different natural sentence would be:

• 参加者がたくさんいます = There are a lot of participants

Both are natural, but they are built differently:

• 参加者が多い uses an adjective
• 参加者がたくさんいます uses a quantity expression plus a verb

Both can relate to many / a lot, but they behave differently.

多い

• an い-adjective
• often used as the main predicate
• example: 参加者が多い

たくさん

• more like an adverb or quantity word
• often used with a verb
• example: 参加者がたくさんいます

So:

• 参加者が多い = There are many participants
• 参加者がたくさんいます = There are a lot of participants

Both are natural, but 多い is the exact word used in your sentence.

By itself, 参加者 does not explicitly mark singular or plural.

Japanese nouns usually do not change form for singular vs. plural.

So 参加者 can mean:

• participant
• participants

The context tells you which is meant.

In this sentence, because of 多い, it clearly refers to multiple participants.

Usually, no.

This is a very common learner mistake.

Even though 多い is an adjective, it usually does not directly modify a noun in this way when you mean many X.

So instead of:

• 多い参加者 ❌

you normally say:

• 多くの参加者 ✅

Examples:

• 多くの参加者が集まりました。
  Many participants gathered.
• 参加者が多いです。
  There are many participants.

So remember:

• 多い works well as a predicate
• 多くの is commonly used before a noun

Because ようです attaches to the plain form of what comes before it.

For an い-adjective, that plain form is just the dictionary form:

• 多い + ようです → 多いようです

You do not change 多い to 多く here.

Compare:

• 高いようです = It seems expensive / high
• 難しいようです = It seems difficult
• 多いようです = It seems there are many

多く is used in other grammar patterns, such as:

• 多くの参加者 = many participants

But not before ようです in this sentence.

A few common casual versions are:

• 参加者が多いようだ。
  Slightly less polite than ようです

• 参加者が多いみたい。
  Very common in conversation; softer and more casual

• 参加者が多い。
  Direct plain statement

So the politeness/nuance scale is roughly:

• 参加者が多いです = polite, direct
• 参加者が多いようです = polite, softer / inferential
• 参加者が多いようだ = plain style
• 参加者が多いみたい = casual conversational

It is read:

さんかしゃ

Breakdown:

• 参加 = さんか = participation
• 者 = しゃ = person

So 参加者 literally means person who participates, or participant.