For official USCF tournaments, they set the number of rounds based on the number of days there are in the tournament rather than the number of players. For a typical weekend tournament (Friday night plus weekend), this will typically be a 5 round Swiss. If there are short time controls (e.g. G/60 or G/45) or it is a longer tournament (e.g., Thursday through Sunday, Friday through Monday), then they might go to 7 or 9 rounds. For one-day tournaments, they usually have short time controls and will have only 4 rounds or possibly 3 rounds. I played a 5 round tournament where there were almost 60 players (high 50s as I recall). In theory, there could have been a tie for first place but I think that there was actually a clear winner in that tournament.
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.
You are either joking, or totally new to chess. Maybe both, but I doubt it.
Of course having more rounds than strictly needed is a good thing. It just make the results more reliable, expecially for those in the middle of the standings.
Are any of you familiar with the McMahon variant of the Swiss system? If more rounds are always better, this seems to be an excellent method that basically creates a virtual extra round. In normal Swiss systems, the first round has the biggest rating mismatches since the upper half is paired against the lower half. If the range of ratings of the participants is wide, then this causes some huge mismatches. Most of the results are predictable (aside from a few big blunders). So with the McMahon system, this first round is basically presumed and the upper half is given a point to begin with. Tournament play then basically begins in the second round. The difference with McMahon and "acceleration" is that the acceleration point is later taken away, while in the McMahon it's kept. I don't think giving and then taking away acceleration points does anything really useful. The San Diego Chess Club has used McMahon effectively for years and I think it's the standard system for the Go tournaments in Europe. I'm surprised it's not more frequently used or recommended for chess.
If you have N players and you want first k places to be fair you need sqrt(N+2k) rounds for that. Another formula is log2(N)+log2(k), where both logs are rounded to the nearest integer.
For 16 players and two places to be accurately determined, you need six rounds; four to determine 1st place and two more for 2nd place.
Do not forget tiebreaker. Else those last a long time i mean a long long time.