Yes, the limit would be once there are no more languages
Addnig language support is not difficult just tedious, as you have to write the interface wrappers.
AMD64 assembly is just as easy as x86. i.e. it's terribly messy but it would essentially be hte same thing, just using the new registers.
I wrote a bit of x86-64 trying to port the ASM implementation of AMX (not the JIT - I'll leave that to dJeyL, assuming he's still working on it).