I really don't know anything about this space

And yet you couldn't help yourself...

but you just said that 8051 is dominant

I said absolutely no such thing.

If the Software can't run because of hundreds of extensions

The software in the x86 world runs because there aren't hundreds of mutually incompatible extensions. I think the last time there was a major completely incompatible x86 ISA divergence was AMD "3DNow" vs other SIMD extensions. AFAIK the rest were "processor X got feature Y later than competitor Z".

Which ISA means my software runs on the most devices?

Easiest question evah: x86.