I don't understand the question . . .
As written, the problem is quite silly.
If a room had to seat 20 adults and their position didn't matter,
is it simply 20! to work out the number of combinations?
I must assume quite a bit that wasn't explained.
I assume: "number of combinations" means number of seating arrangements.
I assume: there are exactly 20 chairs in the room.
I assume: there will be one person seated in any one chair.
Since their position doesn't matter, any person can sit in any chair.
Therefore, there is one seating arrangement.