Averaged over time, one erlang of telephone traffic occupies exactly one channel.
However, the arrival and closing of telephone calls are a random processes. As time elapses, one erlang of traffic may occupy zero, one or multiple channels. The definition of the unit erlang does not say anything about how the traffic behaves statistically about this average.
Thus one erlang of traffic can be generated for instance by
One call of infinite duration, or
A random process of many calls arriving and closing, such that the average number of active calls is one.
The unit of telephone traffic is called after Erlang, a Danish mathematician, who published in 1914 and 1917 the first basic results on the number of subscribers that can be served with a given number of channels at a required Quality of Service (blocking probability).