Angenommen, dass viele kommen, bestuhlen wir den Saal.

Here angenommen means something like assuming, suppose, or let’s assume.

It introduces a condition or hypothesis:

• Angenommen, ... = Assuming that ...
• Angenommen, viele kommen, ... = Suppose many people come ...

So it is not being used here as a normal past participle meaning accepted. It functions more like a discourse marker that sets up an assumption.

Dass introduces a content clause: that many are coming.

However, in this sentence, many learners will notice that Angenommen, dass viele kommen, ... sounds a bit less natural than the more usual:

• Angenommen, viele kommen, bestuhlen wir den Saal.

So:

• with dass: understandable, but less idiomatic here
• without dass: more common after angenommen

A native speaker would very often prefer the version without dass.

Because dass creates a subordinate clause, and in a German subordinate clause the conjugated verb normally goes to the end.

So:

• dass viele kommen
• literally: that many come

This is standard German word order after subordinating conjunctions such as:

• dass
• weil
• wenn
• obwohl

Because German main clauses follow the verb-second rule.

The first whole chunk is:

• Angenommen, dass viele kommen

That entire clause takes the first position in the sentence. So the finite verb of the main clause must come next:

• bestuhlen wir den Saal

This is the same pattern you see in sentences like:

• Wenn es regnet, bleiben wir zu Hause.
• Morgen fahren wir nach Berlin.

After an introductory element, German often puts the verb before the subject.

Bestuhlen means to furnish with chairs, to arrange chairs in, or to seat with rows of chairs.

It is a fairly specific and somewhat formal verb, often used for rooms, halls, event spaces, and venues.

So:

• den Saal bestuhlen = to set up chairs in the hall

In everyday speech, people might also say something more explicit, such as:

• Stühle in den Saal stellen
• den Saal mit Stühlen ausstatten

Because der Saal is the direct object of bestuhlen, so it takes the accusative case.

• nominative: der Saal
• accusative: den Saal

The verb bestuhlen takes a direct object: you bestuhlst einen Raum.

Viele here means many people. The noun is left out because it is understood from context.

So viele kommen is short for something like:

• viele Leute kommen
• viele Gäste kommen
• viele Besucher kommen

German often allows this kind of omission when the meaning is clear.

German very often uses the present tense to talk about the future, especially when the context already makes the time clear.

So:

• viele kommen
• bestuhlen wir den Saal

can refer to a future situation.

English does this too in some contexts, but German does it even more freely. A more explicitly future version would be:

• Angenommen, viele kommen, werden wir den Saal bestuhlen.

But the present-tense version is completely normal.

Yes, the comma is needed.

There is an introductory clause or clause-like expression before the main clause, and German separates this from the main clause with a comma:

• Angenommen, dass viele kommen, bestuhlen wir den Saal.

If you use the more idiomatic version without dass, the comma is still there:

• Angenommen, viele kommen, bestuhlen wir den Saal.

Yes. A very natural alternative is:

• Wenn viele kommen, bestuhlen wir den Saal.

You could also say:

• Falls viele kommen, bestuhlen wir den Saal.

The nuance is slightly different:

• wenn = if/when, neutral condition
• falls = if, often a bit more tentative
• angenommen = assuming/suppose, more hypothetical or more like setting up an assumption

So angenommen sounds a little more like let’s assume this scenario.

It is understandable, but many speakers would find the version with dass somewhat unusual here.

The most natural version is probably:

• Angenommen, viele kommen, bestuhlen wir den Saal.

Or, if you want a simpler and very common conditional sentence:

• Wenn viele kommen, bestuhlen wir den Saal.

So the original sentence is useful for grammar, but in everyday German, most people would probably choose one of those alternatives.