I would say that it doesn't stack, but that's just me. Once you're on fire, you can't be more on fire, really. It's like... jumping into lava and climbing out. In lava, you take 20d6, but out of lava, the residual lava does 10d6 a round to you. If you jump in twice and climb out twice, the residual lava isn't doing 20d6 to you, and it wouldn't do 100d6 if you jumped in and out 10 times.
It's a gray area. RAW I think you'd get 5d6/5d6. As a GM, I'd probably add one dice per doubling of fires. Yes, you can increase the surface area that's burning (the 1d6 assumes bits of you are on fire), but it's not a 1 for 1 increase. So, I'd do...
1-1d6 fire
2-2d6 fire
4-3d6 fire
8-4d6 fire
16-5d6 fire
I would say that it doesn't stack, but that's just me. Once you're on fire, you can't be more on fire, really. It's like... jumping into lava and climbing out. In lava, you take 20d6, but out of lava, the residual lava does 10d6 a round to you. If you jump in twice and climb out twice, the residual lava isn't doing 20d6 to you, and it wouldn't do 100d6 if you jumped in and out 10 times.
There goes my idea of Big B's Lava Dunking Fist.
I'd have the damage stack. It's not like 1 flask is enough to coat your entire body, so multiple flasks = more coverage = more damage.
Agreed on damage stacking. It plays out well with what I call the "15 Point Theory" which is that a robust average human has a grand total fo 15hp, usually 5hp and a death threshold of 10, thus any attack that does 15+ damage is an immediately fatal attack, or the equivalent thereof. Dead on impact.
So, 2d6 damage from alchemist fire, average 7, is enough to render the average person unconcious and on their way to death (which is why most would stop-drop-and-roll to put it out before the second 1d6). Two vials would be 4d6, average 14, almost enough to kill flat out. Three vials is 21, pretty much a guaranteed kill. Thats the equivalent of being completely doused in gasoline and lit up. And thats merely over a 10 second time period.
And if you think you could survive longer than 10 seconds covered in gasoline and lit ablaze, I would highly doubt it. My dalliances with pyromania in my childhood taught me quite quickly that an insulated glove, like the kind you would wear in the snow, soaked in a flammable fluid and lit ablaze will get warm in about 2 seconds, hot in four, if the glove itslef hasn't burned/melted, your hand is cooking inside it by 6 seconds. If insulated and thick material is only buying you about 5 seconds of time, yeah, I can see three vials of alchemist fire killing a person dead in 10 seconds.
And for humor's sake: A flask is what, a pint? You could easily cover a human's body with a pint of liquid, it would just take a good amount of time and effort. Like a massage. A nice, hot massage. But yeah, as a throw contanier which shatters to release it in a splash, its not going to come even close to covering the whole body.
I'd be against it. or else your party is just going to have the 22 strength fire tossing kegs of the stuff at your enemies. (kept safely in a portable hole)