for upland hunting [grouse or pheasants] I wear a vest which has a place for shells in the pocket flaps. [think Bob Allen]
my duck hunting coat has loops in the pockets, and inside the inner jacket for shells. [browning/Columbia]
for deer hunting I just toss a rifles refill of extra rounds in my waist pack which also has water bottles and other essentials in it for the day. [just a three compartment fanny pack type thing]
I also use it to hunt grouse in the mountains and doves and it will hold a box or two of shot shells and a couple of bigger birds no problem.
if I am back pack deer or elk hunting I take a whole box of ammo because I'm gonna walk back in for a few days to a week and I will be mobile from there, so a few extra rounds may be needed.