I don't think there is a perfect magic number. It depends on the team, pitchers, playing positions, parents desires, etc.
In 2005 we had two studs, three pool play pitchers. In 2006 one stud of 2005 gave up pitching, the other had difficulties with the mental game struggled early in the season but started coming back at the end of the year. Two of the 2005 pool play pitchers stepped it up and became the studs of 2006, two more stepped it up and became the pool play, of which one young one moved into elimination games by the end of the season.
ALL OF OUR PITCHERS play other positions, and not just one position. All players on our team are taught as many positions as possible to make our depth strong and the overall knowledge of the game better throughout.
Who knows where and how they will be in 2007 as far as pitching. There is never a #1, #2 or #3, it depends on the opponent, mood of the day, heat of the weekend, rain or shine, etc. Maybe the defense is overall stronger with the #3 pitcher on the mound.
We have three of the five 2006 pitchers returning for next year (one moved up to 16s and one went back down to 12s), we added two more that we look forward to having a part of the action, and yes they will learn other positions. No promises are ever set, purely work and effort and teamwork.