Obviously, the first thing to consider when evaluating a pond is the amount of water
and the type and quantity of filtration attached to it. If we say that for a given
amount of filtration, you can stock 50 inches of mature Koi per 1,000 gallons of
water, it seems like a nice tidy figure. However, if the pond was only of 1,000 gallons
capacity, it would be clearly "overstocked" when asked to hold two 25" Koi.
It would not be overstocked from the point of view of filtration, but its small size means
that the Koi would have no room to move or exercise correctly, and would not thrive.
On the other hand, a 1.000 gallon pond would provide more than adequate space for a lot
more than 50" of small Koi, and would be the sort of environment where they could be grown
on rapidly and successfully. However, you would have to begin to thin them down in numbers
rather quickly as they grew to prevent overstocking by default.