An important question, right?
Given the overwhelming evidence it's highly efficacious (88 studies, 48 peer reviewed, 51 with results comparing treatment and control groups ( links to all at https://c19ivermectin.com, including meta-analyses by meta-analysis specialists working for Cochrane and the WHO )) it's saving many lives. But how many?
If we knew how much ivermectin was being used by humans, we would have a good start. But I can't think of a good way to get that number.
One could extrapolate from the CDC-published data on ivermectin Rx trends in the US, where it is FDA-approved, and off-label prescription and use is legal, and reliable Rx data is collected. They show a large, rapid rise in Rx.
But this data is only for the US and is out of date. Also, a significant fraction of ivermectin use is of product for use on animals, as most physicians are unwilling to #WriteForIvermectin because of the stance of standards bodies. Ivermectin is advocated as best practice in several countries and by several associations and standards bodies. Organizations with the most power (NIH, FDA, WHO, CDC, EMA) have so many censored, ignorant, misled, or corrupt officials that they provide disinformation that reduces use, however. In several nations, use has been prevented by force, so use and sale has been entirely black-market.
Perhaps production data is available from manufacturers, but I'm not aware of any that has been collected.
But even assuming we knew how much ivermectin is being used by humans, it's a hard question. Let's guesstimate a billion people are using ivermectin. Yes, we could be way off, but not by more than an order of magnitude. I expect most of them are in countries where it is given by mass distribution to the entire population, and they don't even know it's preventing CoViD-19 deaths. Officially, around 4 billion human doses have been administered.
The data on CoViD-19 prevalence in these countries could be matched to similar case control countries / populations and the difference in number of deaths could be calculated from public data.
Eight mechanisms of action of ivermectin in the body have been identified. While ivermectin, at the individual patient level, is powerful, its greatest impact on mortality (and societal collapse) is the impact is has on R, the reproduction number, thereby saving the lives (and livelihoods) even of people who didn't take it, due to (temporary) herd immunity effects. At the individual patient level, ivermectin has prophylaxis effectiveness of around 90%, depending on dosage and trial. In those who didn't take it for prevention (PEP or PrEP) and (conservatively) the 10% who who get infected anyway, it reduces mortality around 80%.
Animal doses are probably in the trillions; most animals raised by humans get it routinely - dogs, cats, farmed fish, chickens, cattle, pigs, sheep, etc. And while the proportion of that medicine that has been diverted for human use is probably small, the amount is probably not. This is surely so because even though it is widely discouraged, officially, it is widely advocated unofficially, and many people and even health institutions use it when can find no other way to get the lifesaving drug. Repurposing is relatively error prone because of dosage errors and lack of oversight.
Before trying to answer the question, with the data I know I have access to, I am reaching out here for more ideas and data. On the other hand, if you have and wish to proselytize your faith in the disinformation that reduces use from bodies of censored, ignorant, misled, or corrupt officials, your input is not welcome. Evidence, including conflicting data is welcome, however.