How can set the maximum limit of selections in a CheckBoxList?

In the properties of CustomValidator1, I can't give checkboxlist1 as the control to validate. It is giving me an error.
I am using wizard steps. Because the next step in the wizard has another checkboxlist which will display values based the first selections and so on.
Thanks

At the same time you able to use customvalidator :D

    protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
    {
        byte check = 0;
        foreach (ListItem item in CheckBoxList1.Items)
        {
            if (item.Selected)
                check++;
        }
        if (check > 3)
        {
            CustomValidator1.ErrorMessage = "Warning !!! ";
            args.IsValid = false;
            return;
        }
        args.IsValid = true;
    }

hi
this client code (js)
    protected void Page_Load(object sender, EventArgs e)
    {
        string control = "chck=0; for (i=0; i < " + CheckBoxList1.Items.Count + "; ++i) if (document.getElementById('"+ CheckBoxList1.ClientID +"$' + i).checked) chck++; if (chck > 3){ alert('warning!!!'); return false;} else return true;";
        Button1.Attributes.Add("onclick", control);
    }

this server side code

    protected void Button1_Click1(object sender, EventArgs e)
    {
        byte check = 0;
        foreach (ListItem item in CheckBoxList1.Items)
        {
            if (item.Selected)
                check++;
        }
        if (check > 3)
            return;
    }