Your first customer account number is the number that your customer's account numbers will start from. You can choose between 1, 1000, or choose 'Custom' if you'd like to choose another number. Please note that you cannot edit a customer's account number after it has been set.
In order to set this up, first, open your system settings and scroll down to click 'First customer account number' from the 'Customer information' section.
You will see a page with tick boxes, click on the number you would like your account numbers to start from. There will be help text telling you what number your account numbers will start from depending on how many you already have. To save your changes click 'Save'.