This takes into consideration the benefits of more users, but not the cost. If the cost of adding one more test user is sufficiently small, it can be optimal to

This takes into consideration the benefits of more users, but not the cost. If the cost of adding one more test user is sufficiently small, it can be optimal to test with lots of them. (Roughly, the economic rule here is that you keep doing something until the marginal cost is greater than the marginal benefit.)