team.rb

class Team < ApplicationRecord
  has_many :users
  has_many :business_industries, through: :users

  belongs_to :coach,            class_name: 'User'
  belongs_to :leader,           class_name: 'User', optional: true
  belongs_to :captain,          class_name: 'User', optional: true
  belongs_to :social_director,  class_name: 'User', optional: true
  belongs_to :trainer,          class_name: 'User', optional: true
  belongs_to :sponsor,          class_name: 'User', optional: true

  validates_associated :sponsor
end

team_spec.rb

require 'rails_helper'

RSpec.describe Team, :type => :model do
  let(:admin) { create(:user, role: Role::Admin) }

  before(:each) do
    create_list(:team, 4, sponsor: admin)
  end

  it "doesn't allow for more than 4 teams for a sponsor" do
    admin.reload
    admin.sponsored_teams << build(:team)
    admin.save
    binding.pry
    expect(admin.sponsored_teams.count).to eq(4)
  end
  
end

user.rb

class User < ApplicationRecord
  has_many :sponsored_teams, class_name: 'Team', foreign_key: "sponsor_id"
  validates_length_of :sponsored_teams, maximum: 4
end