Let's take a happy medium between 13 and 17, say, 15. That sounds like a good number to me. I'm sure there are all kinds of competing factors leading to the adoption of a number.
Factors that call for a large number: money, worldwide dispersion (every region wants a tourney), ... please, guys, add the rest.
Factors restricting the number: physical wear and tear on players (ie, health), audience saturation, logistics, ... add the rest.
Short of an exhaustive data reduction through computerized analytical tools, I'll take 15 as this suits my faves
and, more importantly, that explodes into a lot of matches for players that reach semis and finals consistently.
- For those who go a long way in each tourney, say, 4 matches on avg (eg Hingis, Vee, ...):
15 tourneys x 4 matches per tourney = 60 matches
- For those who take a hike on avg after the second round (eg Anna K, ...):
15 x 2 = 30 matches
So the last group may have to enter about 30 tourneys in order to accumulate the same 60 matches. Now, you get the picture given that the physical wear and tear is not related to tourneys but to number of matches.
I'd better stop here, the powers to be are not listening anyway.