To restrict the user from adding more than one line in a Many2many field in Odoo, you can use a Python constraint on the model to validate the number of selected lines. Here's an example of how you can achieve this:

from odoo import models, fields, api
from odoo.exceptions import ValidationError

class MyModel(models.Model):
_name = 'my.model'

many2many_field = fields.Many2many('other.model', 'my_model_other_model_rel')

@api.constrains('many2many_field')
def _check_many2many_field_lines(self):
for record in self:
if len(record.many2many_field) > 1:
raise ValidationError('You can only select one line in the Many2many field.')


In this example, `my.model` represents the model where you have the Many2many field, and `other.model` is the model that the Many2many field is referencing.

The `@api.constrains` decorator is used to define a constraint on the `many2many_field` field. In the constraint method `_check_many2many_field_lines()`, we iterate over the records and check if the number of selected lines is greater than 1. If it is, we raise a `ValidationError` with an appropriate error message.