If 5 noble knights are to be seated at a round table, then how many di

In case nothing is mentioned, in a circular arrangement, two seating arrangements are considered different only when the positions of the people are different relative to each other.
That is, in the question above, you will use the formula (n - 1)!

Here is the theory behind it:

Arranging 3 people (A, B, C) in a row: Following 6 ways
A B C, A C B, B A C, B C A, C A B, C B A
3! ways

Why is arranging 3 people in a circle different?
Look at one arrangement of 3 people A, B and C around a small round table O. Ignore the dots.
....A
....O
B......C
If I am B, A is to my left, C is to my right.

Look at this one now:
....C
....O
A......B
Here also, if I am B, A is to my left and C is to my right.
In a circle, these are considered a single arrangement because relative to each other, people are still sitting in the same position. This is the general rule in circular arrangement.
You use the formula n!/n = (n - 1)! because every n arrangements are considered a single arrangement. e.g. if n = 3, the given 3 arrangements are the same:
.....A ................ C ............... B
.....O ................ O .............. O
B........C ........ A ..... B ..... C........ A

In each of these, if I am B, I am sitting in the same position relative to others. A is to my left and C is to my right.
and these three are the same:
.....C ................ A ............... B
.....O ................ O .............. O
B........A ........ C ..... B ..... A........ C

Here, if I am B, C is to my left and A is to my right. Different from the first three.
Hence no. of arrangements = 3!/3 = 2 only

You might need to use n! in a circle if they mention that each seat in the circular arrangement is numbered and is hence different etc. Then there are just n distinct seats and n people. If nothing of the sorts is mentioned, you always use the (n - 1)! formula for circular arrangement.