This should have the info you need.
The appearance of White Forest is dependent on the number of people
currently residing there. When the player enters the forest for the
very first time, the total number of residents is determined by the
number of days it took the player to reach White Forest. The number of
residents can be between 0 (low occupancy) and 10 (maximum occupancy).
In addition to visual changes, some wild Pokémon will only appear with
a certain resident.
White Forest has a maximum population of 25, although only 10
residents can be present at any one time. Each NPC has a set number of
points, which will change depending on certain functions that can be
performed by the player. If an NPC's points value reaches 0, then they
will leave White Forest. The factors involved in this are as follows:
1 day passes -5 points
Player enters area +3 points
Player talks to character +10 points
New NPCs can be brought into White Forest by inviting them from Black
City via the Entralink. NPCs that have left will be visible to other
players in Entralink. If this area becomes full, no more will leave,
even if their score reaches zero.