As far as I can recall, when the process dies the number in practice is
>3 because of the pmap remove calls for all invididually mapped segments - at least text, data, bss (as you suggest) and probably in practice several others too.

The problem is on pmap remove having to spin rounds loads of stuff and
try and optimise the removal several times. FreeBSD has a 'remove all
mappings for this pmap' call which would do nicely on process exit; as
an aside, on x86 this has the nice effect of a partial cache flush
because of the tables, so such a call wins there too :-) So then you
could reduce the flushes to 1. Beyond that, there's some other changes
you can do to optimise context switching but I can't remember what
happened to those patches (I did 80% of the work one time but there were
a couple of holes and it never made it back to the repository).