If my rifle uses detachable magazines, then I have the one for the rifle full, with the spare mag in my pocket full, and a cartridge carrier on my belt full (8 or 10 depending on carrier). I will also have a spare box along should I need it. If no DM, then magazine full, and carrier on my belt.
Have run out of ammo myself when needed once, and did not like it. HAve also had hunting partners run out of ammo, and needed to finish their animals because they came hunting with just 2 or 3 rounds. Nothing worse than having to go back to get more ammo, and then retake up the trail on a wounded animal that proved tougher than usual.
Some animals, are individuals with tougher constitutions, and even when well hit, do not want to give up the ghost with one or more shots. (My first caribou took 5 in the vitals before expiring, and caribou are not known for being overly difficult to kill; the rest over the years have usually been one and done)
Then there are the times when a bullet gets deflected by an unseen twig, or the animal moves as you finished squeezing the trigger. Anything can happen, and not being prepared because you are too worried about weight of extra ammo on you during a hike or stalk, is just irresponsible. Hope and plan for the best, but be prepared for the worst.
Then there are those of us, who take time out to practice in the field during the day, stump/or rock shooting to hone skills, when taking a break form the hunt (usually in a different area than where we are actually hunting), so extra ammo is always handy.