Hi SmallBizMiss,
Welcome to the QuickBooks Community. Thanks for being a long-time QuickBooks customer. I can provide you with the choices when creating a new company file.
To lower the number of customers in your file, you can create a new company file from an existing one or export the information using IIF. You can pick what comes over and only include the information you want in the new company. If you'd like to review this process, check out the following article: export, import and edit IIF files.
You can create a company based on an existing file. This brings balances and lists such as customers and vendors from your old company file. To complete this, go to File>Create Company from Existing Company File. You then select the current file and create a name for the one you're creating.
This works when best done at year-end, so you begin the year with fresh books.