You simply fill in the number of liters she uses and they calculate. At a two, your wife will probably only need 2 batteries, but I like to err on the side of caution. At this point we travel with 6. Bill uses 4 liters. A battery only lasts at that power about 2.5 hours. Therefore Delta normally says 3 batteries for a 4 hour flight. However, anything can happen: sitting on a runway, slow going thru customs, etc. Better to be safe than sorry. When you do get it to pring, print several. That way you will have them ready to fill out for the next time!
You do have to scan in the completed form and send it. You might want your grandkids to do that and show you how if you don't already know it.