ACT works off the no. of iterations , once it reaches
those it stops. ie. if you have 1000 in the 'Run test a
specific amount of times' box it will run that specific
script 1000 times. The amount of users it will use to
reach it's goal depends on how many simultaneous browser
connections (SBC's) you enter.

If you set the SBC's to 1000 then the when you kick off
the test ACT will attempt to run 1000 instances of the
script all at once, with a new user for each script (so
you would need 1000 users setup). If you only had 1 user
in the SBC's box it would start with 1 script running,
then when it finished it would start the script again
until it had achieved it's goal of running the script 1000
times. SO the load would be very small as the server only
gets hit with 1 user at a time, compared to 1000 users at
roughly the same time.

Hope that helps a bit.