[AJF86 (OP)]

Say that 100 humans (or whatever animal) were to perish simultaneously.  How many of them would become fossilized?

[tommya300]

Say that 100 humans (or whatever animal) were to perish simultaneously.  How many of them would become fossilized?

If all 100 humans (or whatever animal) were to perish simultaneously, and they were in the same pile either 99.9% or non will fossilize.
If a lava flow flooded them it may be different

[Evie]

First of all, you need very specific conditions for fossilization to occur.

Secondly, almost all fossils are formed in sedimentary rock (so a lava flow wouldn't help you at all; it would just destroy the organic material).

If the 100 bodies were all in the same place at the same time (basically assuming under the perfect conditions for fossilization), then they would fossilize at basically the same rate. This is usually a process that occurs over millions of years.

You could also preserve the bodies by dessication (drying out, basically mummified) or freezing (like a mammoth), though. Again, very specific conditions are needed.