id say get four. two albino and two common. espcially if your going to increase the size of the pond.
also stock the pond with small goldfish and gambushia>sp? the cats love them and it'll cost you less in the long run on food.
also only feed them about twice a week. they'll keep the pond cleaner and free of frogs, mice, and bugs if there forced to find their own food. but while their in the tank feed them as much as you like, just dont over do it.
also putting floating plants like lillies, water lettuce (and i forget what the other one i had in my pond was, it was green, about 12" tall and had big bulbs at the base that they floated on and the roots hung down, very colorful when they bloom.) in there will help keep them from jumping out and they'll be more active during the daytime,