Hello Erich, 

You can do this by creating a new calculated field which checks changes on the partner_id.bank_ids. The specific code for counting the banks would be:

for record in self:
  total = len(record.partner_id.bank_ids)
  if record.partner_id.parent_id:
    total += len(record.partner_id.parent_id.bank_ids)
  record["x_bank_count"] = total

Set your dependencies as:

partner_id.bank_ids, partner_id.parent_id.bank_ids	

To create a new field you can go to Settings->Technical->Models search for "sale.order" add a new line on the model. 

If you wanted to change to a boolean, you would just check if the total is > 0. 

Then just inherit the form view and add to the sales order form using:

<?xml version="1.0"?>
<xpath expr="//field[@name='team_id']" position="after">
  <field name="x_bank_count"/>
</xpath>

If you need anything clarifying let me know,

Thanks,