I could have sworn this was already there, but I see it's not.
Agree, this is an essential feature and pretty basic. I would suggest totals at the top of each collection page as a summary statement. The list view could be at the bottom of each column and if other fields get added then some might be average (for ratings or qty)
It's sort of available. I'm guess that the new box of Top Collections on front page shows the number of decks. But I agree that there is no way of seeing that number from the collection view and if implemented it will be handy