The problem is the generated PWM cannot fix 20ms for a period. When the pulse width increase the period also would increase if I control more than 5 servo motors. For less than 5 motors, everything is fine.
And why is that a problem? A servo is only concerned about and responds to the actual width of the pulse which is nominally in the range of 1 to 2 millisec. The fact that the period of the servo frame might vary somewhat from a nominal 20 millisec should not cause the servo any effect.
So what problem are you really trying to solve?
Lefty