'估计是这意思的吧
Option Explicit
Sub abc()
Dim i, m, t
For i = 2 To Cells(Rows.Count, "b").End(xlUp).Row
t = Trim(Cells(i, "b").Value)
If Len(t) Then
If IsNumeric(Left(t, 1)) Then
m = m + 1
Cells(i, "a") = m
Else
Cells(i, "a") = Empty
End If
Else
Cells(i, "a") = Empty
End If
Next
End Sub