Hi.  Seems I recall some rule about sending out only ONE screenplay at
a time and awaiting a response before submitting another.  Is that
true.  Thanks - Denece

Do you mean to agents or to production companies?  Most production
companies these days will not look at a screenplay unless it is
represented, so I assume you mean the former.  Yes, it is the
generally accepted practice to send out a screenplay to one agent at a
time; then, if it is rejected, to query another agent.  As a practical
matter, though, that means it takes a very long time for several
agents to see the play; thus, it is becoming acceptable to advise in
your query letter that you are making multiple submissions.  I always
felt that was kind of cheesy, but the alternative is to send out
multiple submissions simultaneously and not say anything, knowing that
the risk is that if you get caught the agents who received your play
and who became aware of your ruse might look less favorably on your
screenplay.

There is a nice article at http://www.wga.org/craft/queryletter.html. 
It seems to suggest multiple queries are OK.  That's great!

There are some good tips here:

http://www.screentalk.biz/art038.htm