I should note that the koi i'm looking at are between 5 and 6 inches.

Serious koi fanciers usually recommend 1000 gallons per koi and minimum 3' in depth.

plenty of people get by with less.

Thank you for considering the strength and health of your koi - there's also the esthetics - if theres enough room fish will travel around the pond in schools and that is truly beautiful to watch. Seems there's a point where they get too crowded and then they just move around randomly. Not sure what that point is in terms of gallons per fish, but I know it when I see it. Maybe it has to do with size or shape of pond too. The worst is when these 2 or 3 foot long fish hardly have room to turn around - that's just sad, like a tiger pacing in a tiny cage. In those cases I wonder why they didn't get some nice comets instead - with long flowing fins and bright colors some have been bred for ponds and look almost identical - but get only 10 -12 inches instead.

I don't mean to sound judgemental about your pond specifically - I'm commenting on the general question of how much pond per fish. I would think 2000 gals should be adequate for at least some large fish.

Along with gallonage other things should be considered
such as adaquate filteration, both mechanical and biological.

Another condideration besides depth is surface
area. Koi love to swim thus the larger the surface area
provided for the the fish to swim in, the healthier and happier they'll be.
I imagine you could support about 6-7 koi, with adaquate
filtration and keen pond maintenance.

I think it's very important to know exactly how much water the pond will hold, and unless you are absolutely certain of the number of gallons, I highly recommend metering the water into the pond before you replace the fish. You can use the water meter in your front yard. Very often, if the pond is professionally installed or if you buy a house with an existing pond, the water amount may be over estimated by the installer or former owner due to shelves, rocks, or the shape of the pond.

On average, the basics that is recommended for backyard ponds is about 250 gallons per koi. However, most people, including myself, do not strictly adhere to that. As Joann mentioned, it's important that koi have sufficient room to swim freely. I've seem some very small ponds with lots of very large koi, where the fish had almost no room to exercise. With good filtration and aeration, the pond can support more fish than the recommended amount, but swimming is their means of exercise, and they will be healthier if they have plenty of room. That is why many people (including me!) end up having to enlarge their pond -- the fish got way bigger than the owner anticipated!

In a 2,000 pond, your koi will probably reach a maximum size of 2 feet in length, so try to picture a herd of 2 foot long fish swimming in your pond, and then decide how many you think it can support.