How To Count Checked CheckBox Inside ItemsControl

I create multiple checkbox using ItemsControl in my WPF. But I need to make a limit by 20 for checkbox that can be checked/ticked by user. How do I can check the checked checkbox?

I tried google as much as I can, and even binding checkbox to multiple command, but none of it is working. Below is my code to get through the checkbox that were inside the itemscontrol. after, ischecked, I were stuck for this problem for 4 hours already

for (int i = 0; i < ItemsControlUnitPerStrip.Items.Count; i++)
        {
            ContentPresenter container = (ContentPresenter)ItemsControlUnitPerStrip.ItemContainerGenerator.ContainerFromItem(ItemsControlUnitPerStrip.Items[i]);
            CheckBox checkBoxChecked = container.ContentTemplate.FindName("CheckBoxUnitPerStrip", container) as CheckBox;
            if (checkBoxChecked.IsChecked == true)
            {
                //iOPC.WriteTag(checkBoxChecked.Uid, checkBoxChecked.IsChecked);
            }
        }