I have been for ages been a firm believer in 20rds for my deer rifle.
Tell ya why , 1st of all I don't find 20rds to be that heavy , and should I let some bullets loose that don't produce meant , "missed shot, checking the guns zero, shooting something else for whatever reason, etc" that still means if I don't restock on ammo upon arriving home , and I hunt the next day , I still have enough ammo to play out the events again if need be. Now I have never needed these 20rds and often not more than the 1 shot per deer, but if I could forsee the future , I would be out betting on horses and not talking to you people. And frankly even after the hunt is over and I have meat , I see no point in carrying an unloaded rifle , so I plan on having ammo that I won't be shooting for the return trip. If I needed to cut weight , there are so many places I could find to do so.
Now if I only have 19rds , unlikly since I handload , I won't loose sleep over it , but 20 is what I like to have , give or take a few.
Pistols for big game get 3 cylinder fulls , 18rds