REINCARNATION = EVOLUTION
Progression by Law
By law, all spirits, proceeding at their own pace, will eventually progress through all phases. A spirit, out of laziness or ill-intentions, may remain stationary for some period, but, thanks to God's merciful support, it never remains in that state permanently. Likewise, spirits never regress in their evolution . The spirits tell us (#s 118 of "The Spirits' Book") that "as [spirits] advance, they understand the things that slow their progress. When a spirit finishes a trial, it learns a lesson it will never forget."
Question # 120 of the "The Spirits' Book" asks, "Do all spirits have to taste evil to arrive at good?" The spirits answered, "not evil, but unawareness" meaning that a spirit who uses its free will to make good choices will progress much faster, but nevertheless, as Kardec later comments (discussion #127), "even those who follow the right road from the beginning do not attain a state of perfection immediately because they still have to acquire the knowledge and experience that goes with this state." He compares this to the development of a child, whereby even the most precocious, mature, and well-behaved child, must still pass through youth to reach adulthood. spirits who follow the right road will be rewarded, however, for they will arrive sooner at perfection; their imperfections will be fewer and their suffering will be less.
Neutrality vs. Perfection
The elimination of a spirit's negative tendencies and "clearance of all debts" represent a significant amount of progress. However, it is not enough for the spirit to reach a point where it simply does no harm to another. The state of true perfection is one in which a spirit has completely developed virtuous and altruistic behaviors- when it is motivated by love and its actions are driven not by self-interest but rather in the interest of the well being of others.
Number of Incarnations
There is no set number, or limit to the number, of incarnations that a spirit must undergo, though, Kardec writes, ("The Spirits' Book"" discussion #191) "A spirit needs, from our point of view, an immense length of time to pass from spiritual infancy to spiritual adulthood, i.e., the state at which its progress is complete." Each spirit, by the use of its free will, ultimately determines the speed of its own progress through each required stage.