It's limited by the size of the temporary stack during exec, and
platform-specific.
The smallest seems to be 100 4k pages (less space for argv and Tos, I
suppose).
Newer variants of the system raise that to the size of the stack segment
(eg, 16 Mbytes),
which is still platform-specific. In fact, glancing at sysproc.c, I'm not
sure why it's limited
to TSTKSIZ given it goes on to allocate a much bigger segment to put them
in,
but even if that could easily be removed, currently that is the limit.