I'm currently installing a 19row one. Because you'll run with a thermo you don't have to worry about overcooling the oil. the only thing to worry about is the oil pump not building up enough oil pressure because of the added bits to the oil setup.
But there are quite a few people running 19rows so the oil pump is definetly up to the job of supplying pressure for that one.
I would settle for a 19row one. maybe smaller.