Highest I have found in a pre mix is 48, 36 is a bit more common but still not easy to find. 24 is common as the highest a vendor sells, a couple max out at 18. I was a 3 PAD smoker and started on 36, still on 36 and occasionally 48 if I am stressed. I don't notice much difference, it's enough to keep me off analogs but that's all it is. 24 and lower may as well be zero nic as far as I'm concerned - does nothing, sets me to wanting an analog for the nic if 24 or lower is all I have.
Everyone is different, I can chain vape 36 or 48 all day with no ill effects (assuming I stay hydrated of course)