Hi. I hope you realize you can get some 5" to 8" Koi now and in 2 years they could be 2 feet long, getting as big as 3 feet long! I also have a 1600 gallon pond. I have common, comet, shubunkin, and fancy goldfish in it, along with 2 Koi. I think if I was just going to have Koi, I would go with 4. 3 would be more ideal, but I like more fish! Now keep in mind if, as you say, you hope the fish will spawn, you will have a big problem on your hands when you have 100 Koi. You will have to kill them, sell them, give them away, or make a bigger pond! I do think your pond is big enough and deep enough for a few Koi. Enjoy.