Hi,

Please refer to the following links:

1. https://www.cybrosys.com/blog/how-to-add-smart-buttons-in-odoo-18

2. https://www.cybrosys.com/blog/how-to-add-smart-buttons-in-odoo-17

Please refer to the code below:

Python

from odoo import models, fields

class ResPartner(models.Model):
    _inherit = 'res.partner'

    project_count = fields.Integer(
        compute="_compute_project_count", string="Projects")

    def _compute_project_count(self):
        for rec in self:
            rec.project_count = self.env['project.project'].search_count([
                ('partner_id', '=', rec.id)
            ])

    def action_view_projects(self):
        projects = self.env['project.project'].sudo().search([
                ('partner_id', '=', self.id)
            ])
        return {
            "type": "ir.actions.act_window",
            "res_model": "project.project",
            "name": _("Projects"),
            "domain": [('id', 'in', projects.ids)],
            "view_mode": "list,form",
        }

XML

<record id="view_partner_form" model="ir.ui.view">
    <field name="name">res.partner.view.form</field>
    <field name="model">res.partner</field>
    <field name="inherit_id" ref="base.view_partner_form"/>
    <field name="arch" type="xml">
        <xpath expr="//div[@name='button_box']" position="inside">
            <button class="oe_stat_button" type="object"
                    icon="fa-folder" name="action_view_projects">
                <div class="o_field_widget o_stat_info">
                    <span class="o_stat_value">
                        <field name="project_count"/>
                    </span>
                    <span class="o_stat_text">Projects</span>
                </div>
            </button>
        </xpath>
    </field>
</record>

Hope it helps.