INTERESTING AND INMPORTANT QUESTION.
LET ME EXPLAIN AND PUT IT IN THIS WAY :
See if your IVR or a Voice Mail system is connected to an EPABX / Switch and say for example u have got a four port IVR or a VMS , and a caller calls on one of the lines , as long as the caller is interacting with the IVR or the VMS , the port/channel will not be free and no other call can land on this port BUT as soon as the caller opts for transfer to the agent or call center , the port is released and free to receive another call on the same port while the first caller could still be talking to the agent.
Now coming back to your question , I would say YES and NO both.
If the caller is holding on in the IVR then it can handle only one person but if the caller transfers to some other application or call center agent , then more than one person / caller can be handled by the IVR on a single IVR port.
Hope this clarifies , in case u have more queries feel free to write to me.I shall be more than glad to answer all your queries.