We have existng files that we than assign numbers to as they become active. I can not figure out how to have the field pull in the next available number when we activate the file. Could someone please advise me if its possible and how

It should be possible with one or more scripts. What you probably need to do is make a script in the file that you're opening (let's call it counterscript), which adds 1 to a counter field (use a global field). Second, in your main file you probably already have a script that opens the file (if not: make one). In this 'opening script' you should add a scriptstap that activates the script in the file (the counterscript). As a result, the counterfield will become higher everytime it is opened by the mainfile.