This uses names like:

INVOICE 01
INVOICE 02
INVOICE 03
INVOICE 04
...

And assumes that the template worksheet is in the same workbook and it's named
Template (and it can be copied):

Option Explicit
Sub testme()

   Dim iCtr As Long
   Dim TestWks As Worksheet
   Dim NameToUse As String
   
   iCtr = 0
   Do
       iCtr = iCtr + 1
       NameToUse = "INVOICE " & Format(iCtr, "00")
       Set TestWks = Nothing
       On Error Resume Next
       Set TestWks = Worksheets(NameToUse)
       On Error GoTo 0
       
       If TestWks Is Nothing Then
           'found a worksheet name that doesn't exist
           Exit Do
       Else
           'already exists, keep looking
       End If
   Loop
   
   Worksheets("Template").Copy _
       after:=Worksheets(Worksheets.Count)
       
   ActiveSheet.Name = NameToUse
       
End Sub