I tried to find an answer for this for LEGO sets in general, and it seems this is a mystery among the wider LEGO community as well.
What is clear is that production numbers are never set in stone and always vary depending on a number of factors. The set’s size and rarity are probably the two factors that most contribute to how many sets are actually made. Smaller sets are easier and cheaper to make, so they usually are made in very large quantities. In the opposite direction, rare or limited edition sets are usually between the 5000 to 20,000 copy range.
But there are a ton of other factors including the set’s popularity, the line’s popularity, the cost to make or ship the set, and the number of sets produced likely fluctuates over the lifetime of a line.
Sadly, there’s no way to get an exact answer on this unless LEGO ever released this information, which they don’t. We’re left to only guestimate, which even then is very hard to do based on all of those varying factors. It’s even harder with Bionicle since it’s now entirely on the second hand market - most of the ways I see that people try and figure this stuff out is by extrapolating based on the shelves at retailers.
It is interesting food for thought, though. Keep in mind that in the grand scheme of things, the Bionicle community is fairly small. The percentage of Bionicle fans that actively buy parts and build is even smaller. And as time goes on, it’s less and less likely that new fans will enter the community. The number of pieces in the world is shrinking, yes, but so is the number of people who actively want them.
But Bionicle was popular in its prime. One of LEGO’s top-selling lines, in fact. That alone is a good metric to make assumptions. I don’t know if the number of sets would be in the millions - it’s possible, as I’ve seen others estimate that some of the most popular LEGO City sets could be in the millions - but I think the high hundred-thousands is likely for a lot of the earlier Bionicle sets. That might dwindle to the tens of thousands later in the line.
But clearly, LEGO produced enough of these sets to meet their demand, even more than the demand. For the time being, I feel like we are currently in a “functionally infinite” state for Bionicle. And it’s likely there are droves of sets we don’t even know about still hanging around - case in point, DuckBrick’s Bionicle Dream video:
One day there will be no more Bionicle pieces, it’s the sad truth. It’s equally sad but true that, one day, there will probably be no more Bionicle fans. But right now, I feel like there’s enough to go around.
