raikansya ga ooi desu.

It is read らいかんしゃ.

• 来 = come
• 館 = building/facility, often a public facility like a museum or library
• 者 = person

So 来館者 means a person who comes to a facility, usually a visitor. It sounds fairly formal.

来館者 is usually used for visitors to a facility or institution, such as:

• a museum
• a library
• an art gallery
• a city office
• an exhibition hall

It is not the most natural word for every kind of visitor. For example, for a store, you would more often hear things like お客さん or 来店客 instead.

So 来館者が多いです sounds natural when talking about a place that can be described as a 館 or similar public facility.

が marks the grammatical subject.

In 来館者が多いです, the thing being described as numerous is 来館者. So the structure is basically:

• 来館者 = visitors
• が = subject marker
• 多い = many / numerous
• です = polite ending

A very literal way to think of it is:

As for the visitors, they are numerous.

More natural English would be There are many visitors or The number of visitors is high.

In Japanese, 多い is an い-adjective meaning many or numerous.

Japanese often says something like:

• 人が多い = There are many people
• 仕事が多い = There is a lot of work
• 来館者が多い = There are many visitors

So instead of using a verb like English there are, Japanese often uses an adjective to describe the quantity of something.

Because 多い is already an い-adjective, and です is added only to make the sentence polite.

• plain form: 来館者が多い
• polite form: 来館者が多いです

You do not say 多いだ.
That is because だ goes with nouns and な-adjectives, not with い-adjectives.

Examples:

• 静かだ / 静かです
• 多い / 多いです

Because 多く is not the correct form to use before です here.

The predicate form is 多い, so:

• correct: 来館者が多いです
• incorrect: 来館者が多くです

多く can appear in other patterns, for example:

• 多くの来館者 = many visitors
• 来館者が多くて… = there are many visitors, and...
• 多くの場合 = in many cases

But for a simple sentence ending in is many / are many, use 多い.

Yes, 来館者は多いです is possible, but the nuance changes.

• 来館者が多いです: focuses on visitors as the thing that is numerous; often a neutral statement or new information.
• 来館者は多いです: makes visitors the topic; it can sound like As for the visitors, there are many, and may suggest contrast depending on context.

For example:

• 来館者は多いですが、職員は少ないです。
  There are many visitors, but few staff.

So が is often the more straightforward choice in a simple sentence like this.

It is natural, but somewhat formal.

Because 来館者 is a formal word, the whole sentence sounds like something you might see in:

• a report
• an announcement
• a business or public-facility context
• written commentary

In casual conversation, people might say:

• 人が多いです。 = There are a lot of people.
• お客さんが多いです。 = There are a lot of customers/guests.

So the sentence is natural, just not especially casual.

Japanese nouns usually do not show singular vs. plural the way English does.

So 来館者 by itself can mean:

• a visitor
• visitors

In this sentence, 多い makes it clear that we are talking about many visitors, so the meaning is plural in practice.

Because Japanese does not always need a direct equivalent of English there are.

English often says:

• There are many visitors.

Japanese often expresses the same idea by saying:

• Visitors are many.

That is exactly the kind of pattern you see in 来館者が多いです.

If you wanted a sentence with an existence verb, you could say something like:

• 来館者がたくさんいます。

That also means There are many visitors, but the structure is different.

Both can mean There are many visitors, but the nuance is a little different.

• 来館者が多いです
  More like the number of visitors is large.
  Slightly more formal and descriptive.

• 来館者がたくさんいます
  More like there are a lot of visitors present.
  Often feels a bit more immediate and conversational.

So if you are describing attendance or visitor numbers in a formal way, 多いです fits very well.

The spaces are just for learners. Normal Japanese is written without them:

来館者が多いです。

Japanese usually does not separate words with spaces in ordinary writing. So if you see spaces in teaching materials, they are only there to make the structure easier to read.