How exactly do i make a field that lets me show how many of my employees are in a certain department ? 

Here is my code 

# -*- coding: utf-8 -*-
from datetime import timedelta
from odoo import models, fields, api

class employee(models.Model):
    _name = 'drs2.employee'

    name = fields.Char(required=True)
    age = fields.Integer()
    height = fields.Integer()
    annoying = fields.Boolean()

    task_id = fields.One2many(
        'drs2.task', 'employee_id', string="Tasks")

    dep_id = fields.Many2one(
        'drs2.dep', ondelete='cascade', string="Department")


class task(models.Model):
    _name = 'drs2.task'

    name = fields.Char(string="Task Name", required=True)
    description = fields.Text()

    employee_id = fields.Many2one('drs2.employee',
        ondelete='cascade', string="Employee")

    dep_id = fields.Many2one(
        'drs2.dep', ondelete='cascade', string="Department")

    start_date = fields.Date(default=fields.Date.today)

    end_date = fields.Date(default=fields.Date.today)

class department(models.Model):
    _name = 'drs2.dep'

    name = fields.Char()

    employee_id = fields.One2many(
        'drs2.employee', 'dep_id', string="Employee")

    count = fields.Integer(compute='count_employees')

    @api.one
    def count_employees(self):
        return search_count(self.employee_id)