Мне всё равно, какой будет номер в гостинице, лишь бы на балконе можно было хотя бы посидеть вечером и слышать прибой.

Because Russian often uses the dative case for the person who feels or experiences something.

So Мне всё равно literally means something like:

• To me, everything is equal
• more naturally: I don’t care / It makes no difference to me

This is a very common pattern in Russian:

• Мне холодно = I am cold
• Мне интересно = I’m interested
• Мне всё равно = I don’t care / It’s all the same to me

So мне is not the subject in the English sense; it marks the experiencer.

In this sentence, всё равно means it doesn’t matter or I don’t care.

It can be used in different ways in Russian, but here the meaning is:

• Мне всё равно, какой будет номер...
• I don’t care what the hotel room will be like...

Literally, всё равно is closer to all the same.

A useful thing to remember:

• Мне всё равно = I don’t mind / I don’t care
• Всё равно by itself can also mean anyway in other contexts, but that is not the meaning here.

Какой будет номер means what kind of room it will be or what the room will be like.

The verb будет is used because the speaker is talking about a room they have not yet seen or received. It refers to a future or not-yet-known situation.

So the idea is:

• Мне всё равно, какой будет номер...
• I don’t care what the room turns out to be like...

Without будет, какой номер could sound more like a label or description without the same sense of future uncertainty.

Yes, номер can mean number, but in hotel language it also means room.

This is a very common Russian usage:

• номер в гостинице = a hotel room
• забронировать номер = to book a room

So in this sentence, номер definitely means hotel room, not a numerical room number.

Because в plus the prepositional case is used for location: in a place.

• гостиница = hotel
• в гостинице = in the hotel

So:

• номер в гостинице = a room in the hotel

This is straightforward location marking.

Лишь бы means as long as, provided that, or sometimes if only.

It introduces the only condition that really matters to the speaker.

So the structure is:

• Мне всё равно ..., лишь бы ...
• I don’t care ..., as long as ...

In this sentence, the speaker does not care about the room itself, as long as one important condition is met: being able to sit on the balcony and hear the surf.

This is a very common thing that confuses learners.

After words like лишь бы, Russian often uses a form with было to express a desired or hypothetical possibility, even when the overall meaning points to the future.

So:

• лишь бы ... можно было ...

means something like:

• as long as it would be possible to...
• more naturally: as long as one can / could...

This does not mean simple past here. It is part of an idiomatic way to express a wished-for condition.

This is an impersonal construction.

• можно = it is possible / one can
• можно было = it would be possible / one could

Russian often leaves the person unstated when it is obvious from context. Here, the implied person is the speaker, or perhaps the people staying in the room.

So:

• на балконе можно было посидеть
• literally: on the balcony it would be possible to sit
• naturally: you could sit on the balcony / one could sit on the balcony

English usually supplies a subject like you or one, but Russian does not need one here.

Russian uses на with балкон:

• на балконе = on the balcony

This is just the normal preposition used with balconies, because a balcony is treated as an open platform or surface-like space.

So even though English says on the balcony, this matches Russian usage very closely.

Посидеть is the perfective form, and here it means to sit for a while.

Compare:

• сидеть = to be sitting / to sit in general
• посидеть = to sit for some time

In this sentence, посидеть sounds very natural because the speaker means something like:

• at least be able to spend a little time sitting on the balcony

It gives a sense of a limited, pleasant activity, not just the general state of sitting.

Хотя бы means at least.

It adds the idea that this is the minimum acceptable thing:

• можно было хотя бы посидеть
• it would at least be possible to sit for a while

So the speaker is saying: the room itself does not matter much; the minimum requirement is being able to sit on the balcony in the evening and hear the surf.

Because Russian often uses the instrumental case to express time in an adverb-like way.

• вечер = evening
• вечером = in the evening / in the evenings / at evening time

This is very common:

• утром = in the morning
• днём = during the day
• вечером = in the evening
• ночью = at night

So посидеть вечером simply means to sit for a while in the evening.

Because слышать means to hear in the sense of ongoing perception, while услышать means to hear/catch something as a single event.

Here the speaker wants the experience of being able to hear the surf from the balcony, not just one moment of noticing it.

So:

• слышать прибой = to hear the sound of the surf
• услышать прибой would sound more like to catch the sound once or to manage to hear it

The imperfective слышать fits much better with the idea of a continuing atmosphere.

Прибой means surf, the sound of waves breaking, or waves breaking on the shore.

In this sentence, it evokes the pleasant sound of the sea from the balcony.

So:

• слышать прибой = to hear the surf

It is a fairly literary or scenic word compared with simply saying волны (waves).

The commas separate the main idea from subordinate parts.

Structure:

• Мне всё равно, какой будет номер в гостинице, лишь бы...

The first comma comes before какой будет номер... because this is a subordinate clause after мне всё равно.

The second comma comes before лишь бы because лишь бы introduces another subordinate clause expressing the condition.

So the sentence is built like this:

• I don’t care, what kind of room it is, as long as...

In natural English we might phrase it more smoothly, but in Russian the commas clearly mark those clause boundaries.