Count the number of checkboxes which are checked

You can work on selection changed event of checkboxList control like this... 
protected void checkBoxCategoryList_SelectedIndexChanged(object sender, EventArgs e)
{
try
{
StringBuilder sb = new StringBuilder();
int count = 0;
foreach (ListItem items in checkBoxCategoryList.Items)
{
if (items.Selected)
{
count++;
}
}
if (count > 10)
{
string result = Request.Form["__EVENTTARGET"];
string[] checkedBox = result.Split('$'); ;
int index = int.Parse(checkedBox[checkedBox.Length - 1]);
checkBoxCategoryList.Items[index].Selected = false;

ScriptManager.RegisterStartupScript(this, this.GetType(), "Hi", "alert('Category can not be select more than 10.');", true);

Or you can set the label value: (As you say in the question)..

Label.text="Complete"

}
catch { }
}