[plt-scheme] style question

I'm writing a text converter that goes through a text file and
replaces parts of it with place-holders that I can fill back in later.

As a result, I have to keep track of the number of parts I've replaced
and increment the count each time I replace a new substring. If I were
doing this as an OO program, I'd create a Converter class and have the
number of parts so far be a field. Doing the same thing functionally,
it seems like I have two choices--either have the counter be a
module-level variable that all the functions have access to, or pass
it around as a parameter to every function that needs it or calls
other functions that need it.

So, here's my question--which is better style in Scheme? If I don't
provide the variable, it's not really *global* in the scary sense that
global variables are bad, but suppose two different processes both use
the same module at the same time. Does each get its own copy of the
variable, or is there really only one copy that the two processes
would share (and possibly confuse each other as a result)? Passing
around the counter seems cleaner conceptually--all the dependencies
are explicit--but it just feels clunky.

Thanks,
Todd