<script language="vb" runat="server">
Sub generate(ByVal s As Object, ByVal e As EventArgs)
Dim i As Integer
' Dim numtexts As Integer
For i = 1 To 10
Dim t As New TextBox()
t.Text = "TextBox" & i.ToString()
t.ID = "TextBox" & i.ToString()
Controls.Add(t)

Next i

End Sub
</script>


error:Control 'TextBox1' of type 'TextBox' must be placed inside a form tag with runat=server.

This looks more like an ASP.Net Question to me.

http://www.asp.net/Learn/ajax-videos/video-286.aspx

Heres a great tutorial video where a guy explains it... you can also trying googling this:
"How to dynamically add TextBoxes to my ASP.Net Form"
or
"Dynamically add textbox asp.net vb.net"
or
"add textbox dynamic asp.net tutorial"

I am guessing that Controls.Add is not the method you want to use to add this textbox. You should probably be doing Form1.add() or something like that,
Good Luck
seanCampbell