Hi the _prepare_home_portal_values method in Odoo13 doesn't have only self argument but in Odoo 15 it has two self and counters.

See the example for Odoo Code:

def _prepare_home_portal_values(self, counters):
    values = super()._prepare_home_portal_values(counters)
    if 'lead_count' in counters:
        values['lead_count'] = request.env['crm.lead'].search_count(self.get_domain_my_lead(request.env.user))
    if 'opp_count' in counters:
        values['opp_count'] = request.env['crm.lead'].search_count(self.get_domain_my_opp(request.env.user))
    return values