contract/contract_variable_quantity/views/contract_template.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <record id="contract_template_form_view" model="ir.ui.view">
        <field name="name">Contract Template Variable Qty</field>
        <field name="model">contract.template</field>
        <field name="inherit_id" ref="contract.contract_template_form_view" />
        <field name="arch" type="xml">
            <xpath
                expr="//field[@name='contract_line_ids']//field[@name='quantity']"
                position="before"
            >
                <field name="qty_type" />
            </xpath>
            <xpath
                expr="//field[@name='contract_line_ids']//field[@name='quantity']"
                position="after"
            >
                <field
                    name="qty_formula_id"
                    attrs="{'required': [('qty_type', '=', 'variable')], 'invisible': [('qty_type', '!=', 'variable')]}"
                />
            </xpath>
            <xpath
                expr="//field[@name='contract_line_ids']//field[@name='quantity']"
                position="attributes"
            >
                <attribute
                    name="attrs"
                >{'required': [('qty_type', '=', 'fixed')], 'invisible': [('qty_type', '!=', 'fixed')]}</attribute>
            </xpath>
        </field>
    </record>
</odoo>