with 13 tournaments, you have to excel everywhere, but your always fresh.
with 22 tournaments, you have to say in excellent shape and it's harder on the body. but you have more points.
i'm sure that if serena plays 22 tournaments, she would get injured or she would lose more matches/confidence & even some motivation (hingis played a lot but she was mentally exhausted and burned out soon after).
22 tourneys cos eventhough you have the points, you still have to be mentally and physically prepared each time.That is not to say that a player who is plays only thirteen tourneys is not, but they have more time to prepare for the tourneys and the surfaces. It's a better assessment of the opposition and time to work on what you need to work on.
I think playing your best tennis for 22 tournies is harder. There are alot more oppotunities for you to lose and get injured, or just get less motivated to play. Saying that trying to hold onto the 1 spot when yu only play 13 is difficult aswell--it means you always have to get at least the semi's and finals. Something martina hingis did and was criticised for it :-(.
You can be as fressh as you would like by not playing alot of tournaments, but if you don't have the game to be numbert 1 you will not be number 1. How many players out there could play as limited a schedule as Serena, and still win virtually every match. If that is the easier way to number 1 then why aren't the other players doing it?
Not sure exactly what your askign b/c you can read the question more than one way.
To me 13 is harder b/c you have to do EXTREMELY well in those tourny's to do it, you don't get any tournys to write off as a bad day ....however maintaining a high win-loss record is harder at 22 tournys then 13.