In this configuration activity we are able to maintain number ranges for customer master data which later we are able to use for assigning them to account groups. You are able to have either externally or internally assigned number ranges.
IMG Path: Financial Accounting (New) -> Accounts Receivable and Accounts Payable -> Customer Accounts -> Master Data -> Preparations for Creating Customer Master Data -> Create Number Ranges for Customer Accounts
On the first screen you will have to choose the desired mode for number range maintenance or display.
Number Status Maintenance