Schön D.…

On the island of knights and liars, a traveler came to visit his acquaintance, a knight, and saw him at a round table with five guests.

- I wonder, how many of you are knights? - he asked.
- Why don't you ask each of us a question and find out yourself, - one of the guests suggested.
- Alright. Tell me each: who are your neighbors? - the traveler asked.

To this question, everyone answered the same.

- Not enough data! - said the traveler.
- But today is my birthday, don't forget about that, - said one of the guests.
- Yes, today is his birthday! - said his neighbor.

And the traveler was able to figure out how many knights were at the table. Indeed, how many of them are there?