Public Class Form1

    Dim i As Integer = 1
    Dim y = 50
    Dim maxH As Integer = 600

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim cmb As New ComboBox
        cmb.Name = "ComboBox" & (i)
        cmb.Location = New Point(10, y)

        i += 1
        y += 24

        Me.Panel1.Controls.Add(cmb)
        If Me.Height < maxH Then
            Me.Size = New Size(Me.Width, Me.Height + 24)
        End If

    End Sub

End Class