Если в метро слишком много людей, лучше пересесть на автобус.

Because в метро means in the metro / in the subway system, referring to the place or environment.

• Если в метро слишком много людей... = if there are too many people in the metro
• ехать на метро = to go by metro

So Russian uses:

• в метро for location
• на метро for means of transport

That is the same kind of contrast as:

• в автобусе = in the bus
• на автобусе = by bus

After quantity words like много, мало, несколько, сколько, Russian normally uses the genitive plural of the noun.

So:

• много людей = many people
• not много люди

Here:

• слишком = too
• много = many / much
• людей = genitive plural of люди

So слишком много людей literally works like too many of people, even though in natural English we just say too many people.

Yes, you often can.

Both are common:

• много людей
• много человек

The difference is mostly stylistic:

• много людей sounds very standard and neutral
• много человек is also very common, especially in everyday speech when talking about headcount

In this sentence, слишком много людей sounds completely natural.

Russian often uses лучше + infinitive as an impersonal structure meaning it is better to...

So:

• лучше пересесть на автобус = it is better to switch to a bus

There is no need to say a separate word for it is. Russian often leaves that kind of subject unstated in general statements.

This pattern is very common:

• Лучше подождать. = It’s better to wait.
• Лучше не спорить. = It’s better not to argue.

The subject is understood generically. In English, we might say:

• you should switch to a bus
• it’s better to switch to a bus
• one should switch to a bus

Russian often leaves that unstated when giving general advice.

So лучше пересесть на автобус means something like:

• it’s better to switch to a bus
• you’d better switch to a bus

If you want to make the person explicit, you can add a dative pronoun:

• тебе лучше пересесть на автобус = you’d better switch to a bus
• вам лучше пересесть на автобус = you’d better switch to a bus / you all had better switch to a bus

Пересесть is the right choice because it means to switch / transfer from one vehicle to another.

In this sentence, the idea is:

• if the metro is too crowded, it is better to change over to a bus

Compare:

• сесть на автобус = to get on a bus
• пересесть на автобус = to switch to a bus from something else
• пересаживаться на автобус = imperfective, more about repeated/habitual switching or the process

So пересесть fits best because this is a single practical action: leave the metro option and switch to the bus.

Because after verbs like сесть, пересесть, переходить to a mode of transport, Russian normally uses на + accusative.

So:

• сесть на автобус = get on a bus
• пересесть на автобус = switch to a bus

But:

• на автобусе is instrumental and usually means by bus, especially with verbs like ехать
• ехать на автобусе = go by bus

So:

• пересесть на автобус = switch to the bus
• ехать на автобусе = travel by bus

With transport, Russian often prefers на after verbs of boarding or switching:

• сесть на поезд
• сесть на автобус
• пересесть на трамвай
• пересесть на автобус

Even though physically you go into a bus, the idiomatic Russian pattern is usually на + vehicle in this kind of construction.

В автобус can exist in some contexts, but with пересесть and сесть for public transport, на автобус is the normal choice.

Because Если в метро слишком много людей is a subordinate clause introduced by если.

Russian normally separates this kind of clause with a comma:

• Если в метро слишком много людей, лучше пересесть на автобус.

If you reverse the order, you still use a comma:

• Лучше пересесть на автобус, если в метро слишком много людей.

So the comma is a standard punctuation rule for если clauses.

Yes. Russian word order is flexible, and you can move parts around for emphasis.

For example, these are possible:

• Если в метро слишком много людей, лучше пересесть на автобус.
• Если людей в метро слишком много, лучше пересесть на автобус.

The meaning stays basically the same, but the emphasis changes a little:

• в метро earlier emphasizes the location first
• людей earlier emphasizes the crowd itself

The original version sounds very natural and neutral.

Russian often uses the present tense in если clauses for general conditions or likely situations, even when English might think in future terms.

So:

• Если в метро слишком много людей... means if there are / if there turn out to be too many people in the metro...

This sounds like a general rule or piece of advice:

• whenever that situation happens, taking the bus is better

That is why present tense is natural here.