Thanks for following this thread, @steventhom. Yes, the Condensed Data Utility will reduce the number of your customers. 

During the process, you'll be asked to choose which unused entries you'll want to remove.

And you don't have to worry because QuickBooks will create a backup and an archive copy before condensing starts. 

For additional insights, you can visit this link to learn more about what you need to do before, during, and after condensing: Use the Condense Data utility.

You can always visit and post again if you have additional concerns. We're always happy to guide.