There is a minimum number of agents that MUST be provided in order for a stable situation to exist. Failure to provide this minimum number of agents would result in an ever increasing call queue length and a high level call abandonment that cannot be analysed using Erlang C.
If you enter a figure lower than this minimum (which depends on calls duration and number of calls), then you will see an error message when you start a calculation, and the minimum number of agents will be entered for you before the calculation commences.
See also