I keep a spare string and stringer in my gear bag back at the farm house but I don't carry one while I hunt. I guess it would be a pain to have to walk all the way back to get it if needed... but it's not too big a deal.
On hunts away from my home territory I carry a couple of spares. They are down in my bigger gear bag somewhere along with the string wax, flat dental floss and some braided fishing line (that Spectra fishing line in 20# test is pretty handy for a lot of stuff!).
This will probably jinx me and I will go through half a dozen this year but, I haven't ever needed a spare string. I have buddies that I go on what we call "Adventure Hunts" (out of state or off our normal beaten path) that bring all kinds of stuff. Extra arrow shafts, fletching jigs and feathers, 2 or 3 bows... LOTS OF STUFF! I guess I go a minimalist route and just bring a few extra arrows, maybe a spare take down bow. Then again I am a pretty conservative (careful might be a better word maybe) hunter and stay away from "risk associated behaviors" that might cause me to need an entire spare dozen arrows or an entirely new bow for a hunt. Maybe I am just hunting on borrowed time. Of all the lessons bad luck has taught me, I guess this one is later on my syllabus.
OkKeith