I'd guestimate the total volume to be about 600 gallon plus...I'd use like a 210 gallon and converted into a W/D filter and maybe about 3-4 of those Eheims..
There's nothing wrong about 20 Eheims but I think you can achieve better or the same results with a huge W/D and 3-4 Eheims.
Eitherway..nice setup !!