There's no upper limit unless you're walking instead of sleeping (or while sleeping). I think 5+ miles is a good target and is consistent with the amount walked by some hunter-gatherer groups like the Hadza. I usually do 6-8 per day. It sounds like a lot, but once your feet/legs get used to it, it's effortless.
To put it in perspective, my mailman walks 15 miles every day that he works.
I know from previous conversations on PH that walking isn't normally considered cardio, even if you're walking a decent distance. I'm sure it's treading the boundary to cardio if you're carrying weights and walking as fast as you can resulting in elevated heart rate, etc.
In my 30s and 40s I considered 10 miles or more a great weekend hike but now in my 60s 3-5 miles is my preferred distance every other day.
My personal strategy for seeing if I am overtraining or overstressed is to measure my resting heart rate when I wake up. If it starts to elevate, then I back off on stressors like working out, staying up late, etc.
I've met others who've seen results with this method too, but YMMV.
Walking is good for you. Do as much of it as you can comfortably. You will know if you hit the "upper limit" of becoming harmful. For the most part the likely upper limits will be influenced more by your choice of terrain and/or foot wear. So the only "upper limit" I see as a possible negative would relate to mechanical break down. But, even this should not be a concern if you can choose more natural terrain to walk on.