Law of Diminishing Marginal Returns
As variable factors (workers) are added, the marginal output falls
Initially, there is increasing marginal returns as:
specialisation
teamwork
However as optimum capacity limits are approached, eventually each additional unit of labour employed adds less to total output than the previous one, due to:
workers queuing for equipment
workers getting in each others way
This will also lead to rising marginal costs, and marginal returns eventually become negative leading to a fall in total returns, total output falls