In VB, you can use the IsLetter function to determine if a character is a letter. Here is an example code to count the number of letters in a string:

Dim str As String = "Hello World"
Dim count As Integer = 0

For Each ch As Char In str
    If Char.IsLetter(ch) Then
        count += 1
    End If
Next

Console.WriteLine("字母的个数为：" & count)

In this example, we first declare a string variable named str and initialize it with “Hello World”. Then we define an integer variable called count to count the number of letters.

Next, use a For Each loop to iterate through each character in the string. Within the loop, use the Char.IsLetter function to determine if the current character is a letter. If it is a letter, increment the counter, count, by 1.

Finally, output the count of letters to the console. When you run this code, it will output “The count of letters is: 10”, indicating there are 10 letters in the string “Hello World”.