A safe maximum dose for an individual completely depends on that person's tolerance. As well as other factors like metabolism, food/drug interactions, pain, environment (for example an unfamiliar environment lowers tolerance) etc (I have known people who have died from their regular dose with no other drugs taken). You should never assume that an amount that is safe for someone else will be safe for you.
As far the highest dose that someone somewhere could possibly take without dying, I don't know if there is a maximum. I have never heard of such thing as a ceiling level on opioid tolerance, usually money is the limiting factor on how high one's tolerance can get, or people just stop raising their dose because they no longer feel high even from huge doses. Some people have built up insanely high habits/tolerances, like over 1000mg+ (or the equivalent in other opioids).
How much an "addict" consumes per day is going to vary GREATLY, depending on how long they have been using, how much they can afford, whether they are constantly chasing the high or are just taking enough to maintain and keep themselves out of withdrawals, etc.
At the height of my opioid addiction I was probably using an amount in other opioids (mostly heroin) that should apparently be very roughly like around 1500mg+ of IV oxy per day, but it took me years to get to that point and I was spending a crazy amount of money. It is NOT a good idea to take those quantities of drugs, even if your tolerance is so high you're able to do so without dying of an overdose.
I hope you don't use all this info about extremely high doses to feel better about the doses you use though, use it as a lesson to not let your tolerance get that out of control. And always remember just because someone with a tolerance can take something does NOT by any means mean that it is safe or that someone else can take that much and not die.