Probably a long 8 was my biggest then I realized how much stair jumping hurt.
As for ones I've witnessed, one was an 11 flat 11 that he landed about mid way down the second 11 set. Worst one by far was (if memory serves correct) a 15 or 20 stair to flat on a first year 250 with a Jr.T. Oh the days when your ability was measured by how high you were willing to drop.
the biggest clearable stair set where i live is 12 but you have to land flatbottom on dirt road. it was a bad idea but a cleared it. now my back wheel has a small hop to it. (i was on my kona which has 6 inches of travel and it was nice and hrd when i did it...still bottomed HARD...won't do it again. still fun though.)
i forgot to mention that i hit it with to much speed and overshot by about 3 feet (i think thats a little less than a meter)
i kinda doing drops to flat just because its not fun, bad for your bike and it doesnt feel smooth. i have gapped a 6 set to 6 set with about 12 feet in between them (maybe a 17 foot send altogether) still that only counts as a six set but eh who cares i can make up for it on dirt
2 stairs, flat for 4 feet, then about 13 or 14 stairs. Usually land front tire on last 2 steps. But landed to parking lot below a few times just cause. haha
The biggest one I did was by accident. I did about an 8 set at U of Guelph and just kept going for the next, By the time I realized how big the drop was it was too late. I didn't actually count the steps but what a rush. Still I hate drops to flat and would not have done it on purpose, I can't afford to replace cranks, frames, bars ect...