Why only 2? Assuming I read correctly your pond is 7'L x 3'W x 3'D right? Here is a pond calculator you can use. I came up with roughly 471 gallons.
http://www.enkoi.com/calc2.php
You would not be able to keep koi which can get at least 2' in length with good care. Goldfish are smaller most get to 10-12" but I have seen some get slightly larger.
How many you can keep would depend on water quality, what is your winter temps, will it freeze over solid, etc... It would be best to understock to allow for breedings. Goldfish breed like rabbits. Once they start they do not stop. You will have plenty in no time. Out of my 5 goldfish I usually have to rehome 2 or 3 dozen min. by the fall.