There is already Field for this 

Activate developer Mode

Edit Sales view

Add Existing Field > Cart Quantity

You can Label that as you like

After enabling the developer model, go to sale on clicking the debug choose fields -> New

create new field with name x_total_qty, field type = Integer, In Base Properties enable Readonly

Advanced Properties -> compute section add code

for rec in self:

    rec.write({'x_total_qty': sum(rec.order_line.mapped('product_uom_qty'))})

after that click debug from sale order form view , click debug and choose View:Form, Inherited views notebook click add a line , give name to it and in the Architecture add code

<xpath expr="//field[@name='payment_term_id']" position="after">

    <field name="x_total_qty"/>

  </xpath>

save and reload form view you can see the newly added field showing the total sale order quantity after the Payment terms or you can adjust the view.