Σήμερα στα αποδυτήρια είχε τόσο κόσμο που περίμενα έξω δύο λεπτά.

στα αποδυτήρια means in the changing room / locker room.

The noun αποδυτήρια is often used in the plural in Greek, even when English might use a singular expression like the locker room. Greek does this with some place nouns that refer to a space or facility as a whole.

Depending on context, αποδυτήρια can mean:

• changing rooms
• locker rooms
• dressing rooms

So in this sentence, στα αποδυτήρια is most naturally in the locker room / changing room.

Because στα is the normal contraction of σε + τα.

• σε = in, at, to
• τα αποδυτήρια = the locker rooms / changing room

So:

• σε τα αποδυτήρια becomes στα αποδυτήρια

This is extremely common in Greek:

• στο = σε + το
• στη / στην = σε + τη(ν)
• στα = σε + τα
• στους = σε + τους
• στις = σε + τις

That is a very common Greek pattern.

Literally, είχε is the past of έχω = have. But in everyday Greek, έχει / είχε is often used impersonally to mean there is / there are or there was / there were.

So:

• είχε τόσο κόσμο
  literally: it had so much people
• natural English: there were so many people

This is especially common with nouns like:

• κόσμος = people / crowd
• κίνηση = traffic
• φασαρία = noise / commotion

So the sentence sounds very natural in spoken Greek.

Yes, κόσμος can mean world, but it also very often means:

• people
• the crowd
• everyone

In this sentence, κόσμο means people or crowd.

So:

• είχε τόσο κόσμο = there were so many people
• είχε πολύ κόσμο = there were a lot of people

This is one of the most common meanings of κόσμος in everyday Greek.

Because κόσμος here works like a collective noun.

Greek often uses singular κόσμος to mean people in general or a crowd, even though the meaning is plural in English.

So:

• τόσο κόσμο = so many people
• πολύ κόσμο = a lot of people

You absolutely could say τόσους ανθρώπους, but that would be a different phrasing:

• είχε τόσους ανθρώπους = there were so many people

The version with κόσμο is very natural and idiomatic.

Also, κόσμο is in the accusative because it is the object of είχε.

τόσο ... που means so ... that.

In this sentence:

• τόσο κόσμο = so many people
• που περίμενα έξω δύο λεπτά = that I waited outside for two minutes

So the full structure is:

• είχε τόσο κόσμο που...
• there were so many people that...

This is a very common Greek pattern:

• Ήταν τόσο κουρασμένος που κοιμήθηκε αμέσως.
  He was so tired that he fell asleep immediately.
• Έχει τόση δουλειά που δεν προλαβαίνει.
  He has so much work that he can’t keep up.

Because Greek often leaves out the article when talking about an indefinite quantity.

So:

• είχε τόσο κόσμο = there were so many people
• είχε πολύ κόσμο = there were a lot of people

This is not about a specific, previously identified group. It is just talking about quantity.

If you added the article, τον κόσμο, the meaning would shift toward the people / the crowd as a more specific group.

περίμενα is the past form of περιμένω = I wait.

A tricky point is that with this verb, the imperfect and aorist/simple past can look the same in the first person singular: περίμενα.

So context tells you how to understand it.

Here, because the sentence refers to a completed waiting period:

• περίμενα έξω δύο λεπτά

the natural meaning is:

• I waited outside for two minutes

So in this sentence, it is best understood as a completed past action.

In Greek, duration is very often expressed directly, without a preposition.

So:

• περίμενα δύο λεπτά = I waited for two minutes
• δούλεψα τρεις ώρες = I worked for three hours
• έμεινα εκεί μία εβδομάδα = I stayed there for one week

Greek does not need για here.

You may sometimes hear για δύο λεπτά, but the version without για is very common and natural when simply stating duration.

έξω means outside.

In the sentence:

• περίμενα έξω δύο λεπτά it tells you where the waiting happened: outside.

Greek word order is flexible, so έξω can move around more than outside usually can in English.

For example, these are all possible:

• περίμενα έξω δύο λεπτά
• περίμενα δύο λεπτά έξω
• έξω περίμενα δύο λεπτά
  (more marked/emphatic)

The version in your sentence is very natural.

No, it is not required, but it is very natural.

Greek often puts time expressions near the beginning of the sentence to set the scene first:

• Σήμερα = Today
• then the place: στα αποδυτήρια
• then the main event: είχε τόσο κόσμο...

So the sentence flows like this:

• Today, in the locker room, there were so many people that...

You could also say:

• Σήμερα είχε τόσο κόσμο στα αποδυτήρια που περίμενα έξω δύο λεπτά.

That is also correct. The difference is mostly one of focus and rhythm, not basic meaning.

Yes. Greek offers several natural alternatives. For example:

• Σήμερα είχε πολύ κόσμο στα αποδυτήρια, οπότε περίμενα έξω δύο λεπτά.
  Today there were a lot of people in the locker room, so I waited outside for two minutes.

• Σήμερα στα αποδυτήρια ήταν τόσος κόσμος που περίμενα έξω δύο λεπτά.
  Today there were so many people in the locker room that I waited outside for two minutes.

• Σήμερα στα αποδυτήρια υπήρχαν πολλά άτομα, οπότε περίμενα έξω δύο λεπτά.
  Today there were many people in the locker room, so I waited outside for two minutes.

Your original sentence sounds very natural and conversational.