As to the need for extra cartridges on a hunt, there is no doubt in my mind that to do otherwise is an invitation to missed opportunities.
Where I hunt in Texas we have multiple tags and often the opportunity to take multiple deer in a single day. I have (more than once) killed three or more deer on mornings hunt. When you throw in the fact that we are also over-run with hogs and coyotes are very common.....it's quite likely that you could run short of ammo, if just the magazine was filled, on any given day.
Back in my early days of hunting I used to carry 4-5 loose rounds in my coat pocket (still do occasionally). Later it was common (at least in my family) to cut down 20-round ammo boxes so that they held 6-8 rounds. We would fold over the cut box and tape it up to create a handy pocket-size box/carrier. The advantage of this was that the ammo type was printed on the box flap avoiding possible confusion (very important when you carry more than one rifle on a day's hunt).
I later went to a leather folding wallet style 10 round carrier that could be carried on your belt (but was most often just dropped in a coat pocket). Extra ammo is, to me, almost a requirement when hunting.
