You can go down two routes
1. Create the sequential number at point of input and store the number in a field.
2. use a running sum function that you need to create your self.
The second option will slow down your system but aleviates the need for an extra field.
Create function call rSum. See below. Save it in a standard module
Code:
Function rSum(AnyPK As Long, AnyTable As String, AnyField As String) As Long
rSum = DCount("*", AnyTable, AnyField & "< " & AnyPK)
End Function
Then in your query
nIndex: rSum([FieldToCount],"QueryorTable","Field")