OK, so how many walkers are still around? Surely many have been killed by the living, lots have had to fallen off cliffs, bridges buildings etc...impaled themselves, walked into the ocean/river or just rotted to the point that they are no longer a threat.
Is the ratio 100walkers- 100walkers -1 human? 1,000,000-1?
I suspect that ratio is "whatever it takes for humans to still be thoroughly outnumbered". Near as we can tell, as long as the brain isn't destroyed the corpse keeps functioning in whatever limited state it can. Presumably, given what we've seen, there is some sort of other biological functions that is severely reducing the decay of the walkers as well. That said, it's a show/comic in which zombies somehow exist, and not in the parasite/fungus/virus/etc. takes over a living host way but in a dead person somehow magically standing up way. I'm not sure a lot of the rules of logic apply here, unfortunately.
That said, here's a (very) rough estimate of how many zombies are left, given what we've seen on the show. So, going by the video's numbers (which do have some fallacies, of course) we had 6.6 billion zombies when the 5,000:1 ratio was established. Rick and company have killed about 1,000 zombies up season 5 on screen according to
this page, now it's worth noting I said "on screen", there are time skips and obvious offscreen kills as people fire rounds into a horde too, so I'm going to bump that up about 50% as a rough estimate of the true kill count, I'm basing this on it being a nice even number and the fact that presumably despite some fairly large jumps in time most of the action happens when we see it. Now, if I recall correctly (I can't find the numbers immediately), there were 1,500 walkers on screen for the Alexandria bonfire, which mostly catches us up on our kill count, giving us a total of 3,000 total walkers. During the negotiations with Gregory the number of Alexandria residents is pegged at 52, which I'll round to 50. Alexandria is probably roughly representative of the population at large, containing a large amount of people that are bad at killing zombies, some that are just okay, and a few (the main group) that are amazing at it.
Taking that all together, if we assume all of those numbers are vaguely correct, then we have a rate of zombie kill equivalent to roughly 60 zombie kills : 1 living person. Multiple that out by the 400,000 or so folks left as mentioned in the video and roughly 24 million zombies dead, but that doesn't put much of a dent into our zombie totals, basically moving it to 6.575 billion zombies left, or they're outnumbered 16,437.5 zombies to 1 living person. That said, I think figuring out a hard number is a bit of a fool's errand since there's just way too much we don't see of zombie life, we just get glimpses of it from our character's perspective.