Minimum space needs for koi are 1000 gals for first fish, and 500 gals for ea fish thereafter - so you may want to rehome 15 of your little buggers. These are fish which reach lengths of 2-3 ft fairly quickly and as you already know, eat a lot and are poop machines. They need at least 4 ft depth for proper exercise.

If you are proposing overcrowded conditions, like what a dealer might have (short term until fish is sold) then you need a specialized set up capable of handling lots of biowaste and aeration needs.... ie beyond the scope of what the non-professionals on this forum can advise - you might try koiphen or some of the other but even there you're going to be reminded of space needs.

FWIW, 5 koi are a good no., once full size wont look sparse at all. You want them to have room to move around and travel together in a school - which you dont get when theres too many crowded together. You might push it up to 7 or so.... but not 20, please.