You could get a 60 qt (15 gal) kettle ... use FermCap or other foam control drops ... and then add a small amount of top-up water to each fermenter to make up for boil-off....
Or you could get an 80 qt (20 gal) kettle ... do full boils, and not have to top up ... but still watch for boilovers.
Or you could get a massive 100 qt (25 gal) kettle ... do full boils, and barely have to even watch or care about boilovers.
The only real deciding criteria are, do you want to top-up after boiling, and do you ever plan to do 5 gallon batches as an occaisonal thing with the same kettle?
I can say that a 5 gallon batch in a 100qt pot may be very difficult to execute, with perhaps excess boiloff or other ill effects. But a 5 gallon batch in a 60qt pot is very, very comfortable and easy.