SerialIncrement ( ) is the function for you !
What is the purpose of the text that appears at the beginning of this text?
One option is to keep the Text in one field and the auto-entered serial number in another. Then for display/reporting purposes, use a calculation field that combines them:
TextField & right ( "00000" & SerialIDfield ; 5 )
The biggest issue with such a numbering system is that your have a built in maximum value beyond which your system fails. In this example, INV99999 is the larges permitted value.