organisation.rb

class Organisation < ApplicationRecord
  NUMBER_OF_PERMITTED_USERS = 10
  has_many :users, before_add: :validate_user_limit

  private

  def validate_user_limit(user)
    raise Exception.new if users.size >= NUMBER_OF_PERMITTED_USERS
  end
end

organisation_test.rb

test 'attempting to add more than the permitted number of users to an organisation should fail' do
    org = create(:organisation)

    assert_raises Exception do
      # Adding 11 users should raise an exception but allow the first
      # 10 to be added without a problem.
      # This assertion catches the exception thrown so that we can run the
      # assertion on line 14 without the test erroring.
      org.users.push(create_list(:user, 11))
    end

    # Check that despite attempting to add 11,
    # only 10 have been added.
    assert_equal 10, org.users.size
  end

user.rb

class User < ApplicationRecord
  # The optional: true flag is only for Rails 5 if a User
  # doesn't always belong to an organisation.
  belongs_to :organisation, optional: true
end