Tally can only currently support numeric (entirely numbers based) invoice numbers and cannot support any letters or special characters and must be unique.
When creating an invoice, the invoice number is automatically inserted in the next available number after your current highest invoice.
For example; the highest numbered invoice number you have in Tally is 99, the next invoice that is created will be automatically produced is 100, regardless of any unused numbers prior.
Invoice numbers should always be sequential. For further information on invoice numbering requirements, have a look at HMRCs guidance at http://www.hmrc.gov.uk/vat/managing/charging/vat-invoices.htm.