VBA For Adding Numbers

I have a sheet that auto-populates with code from a userform that also provides the cell formatting. What I need is that in Column A it keeps a running count of the errors that are populated on the sheet.

Code for the formatting


With ActiveSheet
lastrow = .Cells(.Rows.Count, "B").End(xlUp).Row

For i = 4 To lastrow

If Cells(i, "B").Value "" Then
With .Cells(i, "A")

With .Offset(0, 0)
.Value = "1"
End With

.Offset(0, 1).Resize(1, 9).Borders.LineStyle = xlContinuous
.Offset(0, 5).Resize(1, 3).Interior.ColorIndex = 35
End With
End If
Next i
End With
What I need is instead of Value of "1" with the offset(0, 0), I need it to take the previous rows value and Add 1.

So the Range("A4").Value = "1"
Range("A5").Value = "2"

As the new errors are added to the sheet I need the value in "A" to be 1 more than the previous "A" value.