To change the invoice numbers that are automatically generated:
Click the Invoices tab, and then click the New Invoice button that is on the right-side of the screen.
In the New Invoice page, next to the Invoice#, click the Change Setting link.
Select the option No if you want to disable the automatic generation of invoice number. This way you'll have to manually enter an invoice number every time you create a new invoice. Choosing Yes will automatically generate an invoice number based on the number you define below.
Prefix refers to the text that is displayed before the invoice sequence number. By default, the prefix in Zoho Invoice is 'INV-'. You can change this to suit your invoice numbering requirements.
Next Number refers to the sequence number of the next invoice.