Why did the number of water molecules on each side of the membrane change?

because of the quantity of solids that are inside or outside the cell.

Explanation:

Remeber that the cell pressure will always tend to make up for the same pressure inside and outside the cell. If for some reason, there is more particles inside the cell, the water molecules will tend to come inside to regulate the inside pressure. In the other hand, if there is a lot of particles outside the cell, the water molecules will live the inside part of it to regulate the surrounding pressure.