Angenommen, dass die Teilnehmerzahl steigt, ändern wir die Bestuhlung.

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

It comes from the past participle of annehmen (to assume), but in this sentence it is being used as a fixed introductory expression.

So:

• Angenommen, ... = Assuming that ... / Suppose that ...

In this kind of use, it introduces a condition or hypothesis rather than a completed action.

Not always. In fact, many learners will more often see:

• Angenommen, die Teilnehmerzahl steigt, ...

without dass.

That version is very common and often sounds more natural.

So these are both possible in principle:

• Angenommen, dass die Teilnehmerzahl steigt, ändern wir die Bestuhlung.
• Angenommen, die Teilnehmerzahl steigt, ändern wir die Bestuhlung.

But the version without dass is often the more idiomatic one in everyday German.

A very common alternative would also be:

• Falls die Teilnehmerzahl steigt, ändern wir die Bestuhlung.
• Wenn die Teilnehmerzahl steigt, ändern wir die Bestuhlung.

Because dass die Teilnehmerzahl steigt is a subordinate clause.

In German subordinate clauses introduced by words like dass, weil, wenn, falls, and many others, the conjugated verb usually goes to the end.

So:

• dass
  • die Teilnehmerzahl
    • steigt

Compare:

• Main clause: Die Teilnehmerzahl steigt.
• Subordinate clause: ..., dass die Teilnehmerzahl steigt.

That verb-final position is one of the most important German word order patterns to learn.

Because when a different element comes first in a German main clause, the conjugated verb must still stay in second position.

The first element here is the introductory clause:

• Angenommen, dass die Teilnehmerzahl steigt

After that, the main clause begins, and the verb comes immediately:

• ändern wir die Bestuhlung

So the structure is:

• First position: the whole introductory clause
• Second position: ändern
• Then the subject: wir

This is the normal verb-second rule in German main clauses.

Compare:

• Wir ändern die Bestuhlung.
• Wenn die Teilnehmerzahl steigt, ändern wir die Bestuhlung.

Same rule.

Because the sentence begins with a subordinate clause or introductory clause, and German normally separates that clause from the main clause with a comma.

So:

• Angenommen, dass die Teilnehmerzahl steigt, ändern wir die Bestuhlung.

The comma marks the boundary between:

1. the assumption/condition
2. the main statement

German uses commas more systematically than English in these kinds of clause structures.

Die Teilnehmerzahl means the number of participants or attendance number.

It is a compound noun:

• der Teilnehmer = participant
• die Zahl = number

Together:

• die Teilnehmerzahl = participant number / number of participants

Even though it refers to multiple participants, the noun itself is grammatically singular, because the head word is Zahl.

That is why the verb is singular too:

• Die Teilnehmerzahl steigt.

not

• Die Teilnehmerzahl steigen.

Because both nouns are feminine singular:

• die Teilnehmerzahl
• die Bestuhlung

Their grammatical gender is feminine, so the article is die.

In this sentence:

• die Teilnehmerzahl is nominative, because it is the subject of steigt
• die Bestuhlung is accusative, because it is the direct object of ändern

For feminine singular nouns, die is the same in both nominative and accusative, so the form does not change here.

Not exactly. Bestuhlung usually means the seating arrangement, chair layout, or the way chairs are set up in a room.

So it is more specific than just chairs.

For example, it can refer to:

• classroom seating
• theater-style seating
• conference room chair arrangement

So wir ändern die Bestuhlung means something like:

• we change the seating arrangement
• we rearrange the seating

Ändern means to change or to alter.

Here it takes a direct object:

• die Bestuhlung ändern = to change the seating arrangement

This is a normal verb-object combination.

Depending on context, German might also use other verbs, such as:

• anpassen = adjust
• umstellen = rearrange
• neu organisieren = reorganize

But ändern is perfectly understandable and grammatically straightforward.

Because in German, numbers, prices, temperatures, and similar things often steigen (rise / increase) by themselves.

So:

• Die Teilnehmerzahl steigt. = The number of participants increases.

This is an intransitive use: the number itself is going up.

If you wanted to say that someone actively increases something, you might use another verb, such as:

• erhöhen = to increase, to raise

For example:

• Wir erhöhen die Teilnehmerzahl. = We increase the number of participants.

But that would mean we are causing it to increase.

Because German very often uses the present tense to talk about the future when the context already makes the future meaning clear.

So:

• Angenommen, dass die Teilnehmerzahl steigt, ändern wir die Bestuhlung.

can refer to a future situation, even though both verbs are present tense.

English does something similar sometimes:

• If attendance increases, we change the seating arrangement.

But English often prefers will in the main clause. German does not need a future form here.

You could use werden for the future, but it is often unnecessary:

• ..., werden wir die Bestuhlung ändern.

That is also possible, but not required.

It sounds fairly neutral to slightly formal, especially because of Teilnehmerzahl and Bestuhlung.

It feels like something you might read or hear in:

• event planning
• administration
• business communication
• meeting organization

A more everyday version might be:

• Wenn mehr Leute kommen, stellen wir die Stühle um.

That is less formal and more conversational.

So the original sentence is not unnatural, but it belongs more to organized or professional contexts.

Yes. A very common and natural version would be:

• Wenn die Teilnehmerzahl steigt, ändern wir die Bestuhlung.

Or:

• Falls die Teilnehmerzahl steigt, ändern wir die Bestuhlung.

Or, if you want to keep Angenommen:

• Angenommen, die Teilnehmerzahl steigt, ändern wir die Bestuhlung.

These all express a similar idea, but:

• wenn = if/when
• falls = if, in case
• angenommen = assuming/suppose

So the original sentence is understandable, but many native speakers would probably prefer one of these alternatives, especially the version without dass after Angenommen.