I think that KellerBrauer has the units correct for the pitching rates. I, too, have never seen a pitching rate at other than billion cells/ml/?Plato. If you go by KellerBrauer's findings and target around 17 million cells/ml you would want to plan for a pitch of 17 mil/ml * 19 l * 1000 ml/l = 323,000 million cells or 323 billion cells. This seems like an overpitch for an ale, but they may do it to lessen the ester production for a cleaner ale character in the finished beer.
Mashing for 20 minutes seems on the short side to me, but it would definitely add to the body of the finished beer. You can always run the mash for 20 minutes and then take a wort sample and check for completeness of conversion using a starch indicator test. If the starch test comes out positive, mash a bit longer and test again.
As with any recipe, I recommend selecting the parameters which make sense to you, brew, and then make adjustments for the next brew based upon your experiences and outcome.