25-30 players should be 5-6 rounds or so.

That's my guess, according to my experiences.

In 9 players, every round will have a person getting a 'bye', but not the same person twice.

In general, the more rounds the better.   As you increase the number of rounds in a Swiss, the more it resembles a round robin.  I think the time available, rather than the Swiss system itself dictates the number of rounds in a tounament.

Yes, the more rounds the better; the minimum for 25-30 players is 5 rounds. The general rule is that x rounds are usually enough to have a crear winner out of 2^x players: 5 rounds is enough for up to  2^5=32 players. 4 rounds is enough for up to 16 players.

Usually it is normal to calculate no. of players divide by 3 to get optimal round number. So for 25 - 30 players it is normal to get 9 rounds in swiss system and all top plyers should play each other.

Think how many rounds would be needed for a tournament of 498 players. 166 rounds?

http://www.cappelle-chess.fr/fr2/default.php?page=3967  or nine?

9 is a normal no. of rounds in a tournament, or 13 (i think ) on chess olimpiad. We ware here talking about round number for small tournament how to calculate.

One way to think of it is that for every round, up to half the players can get 1 point.  So for 30 players, after rd 1, there can be 15 with 1 point, then 8 after rd 2, 4 after rd 3, 2 after rd 4 and finally 1 after rd 5.  

If you have a calulator with a ln (logarithm) function you can calculate:

rounds = ln(players)/ln(2), then "round" up.  (no pun intended!)

or, rounds = log2(players) (log2 = base 2 logarithm).

Or use a table like this where for each round, you double the maximum number of players:

1 rd, 2 players

2 rds, 4 players

3 rds, 8 players

4 rds, 16 players

5 rds, 32 players

6 rds, 64 players

and so on...

Of course, if you have time for an extra round or two, more of the top players will have a chance to play each other.  

Your post seems to indicate that you thought 9 rounds was adequate for 498 players.  Your link shows that places 1-7 had identical scores of 7.  The next 15 places  (6-21) were half a game off the pace with 6.5 wins.  I imagine a good number of the top 21 didn't lose a game.  How many of the top 21 got to play each other in 9 rounds?

You could have a 166 rounds and everyone could be tied at 83 points. A minimum number of rounds in a swiss style does not guarantee no ties. It guarantees there are no tied players who won all their games.

2 to the power of 8 is 256, 2 to the power of 9 is 512 so nine rounds is the minimum to determine a winner of a swiss style tournament of 257 to 512 players if two players dominate the others.

Round 1 - 512 players tied with 0 points

Round 2 - Maximum 256 players tied with 1 point

Round 3 - Maximum 128 players tied with 2 points

Round 4 - Maximum 64 players tied with 3 points

Round 5 - Maximum 32 players tied with 4 points

Round 6 - Maximum 16 players tied with 5 points

Round 7 - Maximum 8 players tied with 6 points

Round 8 - Maximum 4 players tied with 7 points

Round 9 - Maximum 2 players tied with 8 points meet in the last round.

If there were only eight rounds the top two players would not have met.

As indicated by previous posters you could have more rounds.

Notice a knockout style tournament would have the same nine rounds.

Notice that a knock out tournament would need more games because of draws, even if you don't call replaying after a draw a new round.  Notice what 9 rounds did to Petrosian who probably didn't lose a game.  If there was a draw after 166 rounds, I would say it was a legitimate result.  After 9 rounds, when few of the top players have played each other I think a draw is unsatsfactory, and going to tie breakers doesn't prove anything.  166 rounds would be impactable because of time restrictions.  13 rounds would have been a vast improvement.  I still say the more rounds the better.  Your example tends to show why 9 rounds is not optimal.

but what if we have 9 players?How many round in that case?Or is it better ,in that case play round robin?

If you have time for 8 rounds then a round-robin. If you have time for 16 rounds then a double round robin.

If you do not have the time then see the post from RoyLupez

...use a table like this where for each round, you double the maximum number of players:

1 rd, 2 players

2 rds, 4 players (3 to 4 players)

3 rds, 8 players (5 to 8 players)

4 rds, 16 players (9 to 16 players)

5 rds, 32 players (17 to 32 players)

6 rds, 64 players (33 to 64 players)

and so on...

The calculation is this:

2 to the power of 3 = 8, so 3 rounds for 8 players.

2 to the power of 4 = 16, so 4 rounds for 9 to 16 players.

If you do not have time for 8 rounds then a Swiss with a minimum 4 rounds works for you.

Also to be considered is how many places need to be accurately determined.  Once first is determined you may need two more rounds to get 2nd, four more to get 3rd.  For team selection tournaments in the late 60's, a round robin was the way to go.  This is what I put together to create round robin schedules a few years ago: http://home.comcast.net/~wporter211/realsite/chess_etc/rrpair.htm